Unless you happen to have been living on Mars for the last year or so, you probably know that this week (January 26 to be precise) it will become a criminal offence (in England and Wales) to possess pictures that the government deems to be "extreme porn". You might also be aware of two diametrically opposed views on this legislation.

On the one hand, soothing words from Government and the Ministry of Justice suggest that the offence will catch only a very small number of people who possess the most abhorrent imagery. Cynics - including many lawyers who know a thing or two about obscenity - point out that the law as written is very broad, could catch a lot of quite ordinary stuff, and remind us of how laws, originally "intended" to focus on the most serious offences, end up being used far more widely. A good example is the Regulation of Investigatory Powers Act 2000, supposedly aimed at terrorists, and now used by local Councils to pursue individuals guilty of anti-social doggy poo.

Some of the advice in this article is therefore specific to the new extreme porn law: some is more general. It is not legal advice: if you want that, talk to a lawyer. It does set out some of the things you need to know, as well as some things you need to do now in order to make sure your story is not on the front pages of The Register in months to come.

First off, just what is the law? Various guides - including an official release from the Ministry of Justice - make it clear that pictures will fall foul of the new law if four components are present. The pic must be pornographic, or produced for sexual purposes. It must be realistic. It must contain certain specific imagery, including necrophilia, bestiality, activity depicting serious harm to breast, anus or genitals or life-threatening activity. Finally, it must be grossly offensive, as determined either by a jury or magistrate.

All those components must be there. Poster your walls with the most grotesque, the most blood-spattered out-takes from Saw or Hostel and unless someone can prove you actually get off on them, no prosecution could follow. That, of course, highlights one of the first of many question marks hovering over this legislation: is "produced for sexual purposes" defined relative to the motives of the originator of an image, or the motives of the person who downloads it to their hard-drive? Lawyers suspect the latter: so in fact, the out-takes in question might or might not fall foul of the law depending on your personal sexual tastes.

There are two defences to a charge under this act: that you were participating with a consenting partner in the scenes depicted (and no law was broken in the creation of those scenes), or that the scenes exist wholly within a BBFC rated film (DVD, video, etc). Both these defences have their flaws. The first does not apply to the person taking the photo - unless they were also participating in the act. Since the law will apply on an image by image basis that probably means that you must include some (identifiable) part of your anatomy in every shot.

The BBFC exemption only applies to whole films: watch A Clockwork Orange, replay the rape scene over and over in the privacy of your living room, you commit no offence. Clip that one scene for viewing on your PC, and chances are that you might.

When it comes to computer crime (any crime, for that matter), the first rule is don't commit the offence. Or if you think you may have inadvertently broken the law, don't get caught. Easier said than done.

A very high proportion of crime is not so much detected as denounced by people's nearest and dearest. Hence all those Crimewatch appeals asking viewers if their partner was acting suspiciously on a particular night. Many companies that come to the attention of the Information Commissioner for Data Protection breaches do so as a result of falling out of love with former employees. IT staff know where the bodies are buried - and there is none so furious as a sysadmin scorned.

Limbering up for a divorce by looking at internet porn sites may provide a degree of self-gratification: but if the end result is going to be a bitter battle over money, maintenance and kids, be very careful. Courts are supposed to work for reconciliation, but there are still lawyers - and exes - out there who might consider an allegation of looking at illegal porn a useful move as part of the separation end-game.

At present, it seems unlikely that there will be an extreme porn equivalent of "Operation Ore". Readers may remember that in that case, British police went after a list of 7,000 individuals named as subscribers to a US-based site acting as an alleged gateway to child porn. While the reverberations of Operation Ore are still being felt within the UK Appeals Courts, the key point is that distribution and possession of paedophile material is seriously illegal not only in the UK and US, but also in most jurisdictions throughout the world.

Subscribe to a child porn site anywhere in the world and do not be surprised, when that site gets busted, if you are shortly after invited to assist the British Police with their inquiries.

That is not quite the case with the law on extreme porn, which has been passed precisely because most of the material it covers is hosted quite legally in other countries. In early debate on the measure, it was estimated that some 80 per cent of extreme porn emanated from sites operating perfectly legally in the US. The British Police might request the names of subscribers to such sites: but it is unlikely that names would be forthcoming.

Beware complacency, however. The British police do have powers under RIPA to demand that ISPs cough up information about what any individual has been downloading in order to "help the prevention of crime". Given the way in which RIPA has been extended, it is not beyond the bounds of possibility that the police might use that law to monitor individual surfing habits.

That would not, by itself, be proof that you possessed an extreme pornographic image. However, a stream of downloads from "Hanging Bitches" would probably constitute sufficient grounds for taking a look at your PC.

While no one in the Ministry of Justice has said as much, it is also possible that the police and Crown Prosecution Service will, in time, invent an additional and complementary offence to go with the extreme porn law. The majority of individuals convicted under Operation Ore were convicted (or accepted cautions) for the crime of possessing indecent images of children, but a small but significant minority were found to possess no images at all.

They argued that this was because they had never downloaded any, and the police evidence was flawed: the prosecution argued that they had simply deleted them successfully. Since they appeared to have subscribed to sites promoting such imagery, the CPS then proceeded to bring charges of incitement to distribute indecent imagery. It is not unthinkable that similar charges could be brought against individuals who had no extreme porn on their hard drives, but who were active subscribers to websites featuring such stuff.

But what if you have been looking at material that you fear might fall foul of the law? The simple answer is that you need to wipe your hard-drive. Some useful tips on how to do so are provided here, together with a certain amount of common sense guidance.