Windows 7 is now for sale around the world. Microsoft CEO Steve Ballmer is officially kicking off the Windows 7 launch at an event in New York City. Microsoft's partners will be unveiling new hardware and OEMs are now no longer recommending Vista to consumers and are switching their default choice to Windows 7. Today is easily the most important day for Microsoft this year (the operating system's RTM date of July 22, 2009 comes in at a close second place). Here's everything you need to know about what's happening today.

The editions

Windows 7 comes in six editions, and while this may seem like a lot, don't worry, you only really have to worry about two: the Home Premium Edition and the Professional Edition. Home Premium is what you want if you use your computer for personal use and entertainment purposes. Professional is what you want if you're in the corporate world—basically anything significant for work. Both are available at retail and with the purchase of a new PC, and Microsoft will make sure to market both like there's no tomorrow.

Starter Edition is only available via OEM purchase, and is shipping only on low-end computers. Home Basic Edition is only available in emerging markets, so chances are you won't see it at all. Ultimate Edition and Enterprise Edition are pretty much one and the same, the main difference being that the former is aimed at computer enthusiasts and the latter is for corporations. You'll find Ultimate on sale at retail and on select OEM computers, while you'll likely only encounter Enterprise if you work for a large company.

Users might also want to know that Windows 7 is available today worldwide in 14 different languages: English, Spanish, Japanese, German, French, Italian, Dutch, Russian, Polish, Brazilian Portuguese, Korean, Simplified Chinese, Traditional Chinese, and Chinese (Hong Kong). The remaining languages will be available starting October 31, 2009.

Get it cheap

Once you've figured out which edition you want, you have to go out and get it. You can do so by buying a new PC, hunting around to find the best bang for your buck online, or at your favorite big-box retailer. Wherever you get it, Windows 7 will be cheaper than Windows Vista. Upgrade retail prices range from $120 (Home Premium) to $220 (Ultimate) while the full retail products range from $200 (Home Premium) to $320 (Ultimate).

Still, we know you want it for less than retail. The solution, as with previous Windows releases, is buying an OEM version of the OS. OEM copies of Windows are cheaper, mainly because they're intended for system builders who install them on new custom PCs. Preliminary OEM prices were revealed in September 2009, and they range from $110 for a copy of Windows 7 Home Premium to $550 for three copies of Windows 7 Ultimate. We expect that they'll go lower than that though, so keep your eyes peeled.

Keep in mind that the license for an OEM copy does not allow you to transfer the operating system from one PC to another (the product key is tied to a single machine), it doesn't come with any support, and only allows a clean installation.

Microsoft today also launched a US-only promotion from today to January 2, 2010 for customers who are looking to get a new PC and still be able to upgrade their current machine. Titled "The Buy a PC, Get a Discounted Upgrade," it lets customers who buy a new PC with Windows 7 Home Premium, Professional, or Ultimate will be able to purchase a full upgrade copy of Windows 7 Home Premium, Professional, or Ultimate at a discount to upgrade an older machine.

The Windows 7 Family Pack, which allows you to upgrade three PCs to Windows 7 Home Premium, becomes available today, for a limited time. In July 2009, Microsoft announced that the Family Pack will cost $149.99 in the US and $199.99 in Canada, which is a about $200 of savings for the three upgrade licenses. In August 2009, eight more European countries joined the deal: the UK (?149.99), Ireland (?149.99), Germany (?149.99), France (?149.99), Switzerland (?149.99), Austria (?149.99), the Netherlands (?149.99), and Sweden (?149.99).

Still not cheap enough for you? We understand. Students can get upgrade versions of Windows 7 for $30 in the US and ?30 in the UK. Students in Australia, Canada, France, Germany, Korea, and Mexico can participate in this offer today as well (exact pricing for those countries has yet to be revealed).

If none of the above seems appealing enough, what about getting Windows 7 for free? Microsoft has already given Windows 7 away to its testers, launch party hosts, Twitter fanatics, Ars readers, and even launch event attendees. All that happy giving happened before the OS was officially available, but a little birdie told us there are more giveaways to come. We'll keep you posted.

Clean install versus upgrade

Before you install Vista's successor though, check to make sure your machine is Windows 7 compatible. Earlier this week, Microsoft released two tools for checking compatibility, and both are great for helping you through the migration process. If you are in the five percent or so of users that Microsoft says actually upgrades their previous Windows installation (as opposed to buying a new PC with the new OS), the traditional received wisdom was to always perform a clean install of any new version of Windows; if you're upgrading from XP, that's the only choice you get anyway. If you're upgrading from Vista, however, the upgrade process works surprisingly well, and can be tentatively recommended as an effective—and conveniently non-destructive—way to install the new platform.

Also, before you take the plunge, make sure to back everything up. Yes, everything—just to be on the safe side (even if you're performing an upgrade install!). If you've only got a single internal hard drive available, go ahead and partition it and move everything there, but an external hard drive or a few DVDs will also do the trick.

A little history

We've come a long way since Windows XP:

OS RTM Official launch date Windows 7 July 22, 2009 October 22, 2009 Windows Vista November 8, 2006 January 30, 2007 Windows XP August 24, 2001 October 25, 2001

Windows XP will be eight years old on Sunday; the majority of the market is still using a very outdated operating system. Also note that the gap between the RTM and official launch dates is widening: with XP it was two months, with Vista it was a bit more, and with Windows 7 it is three months. This poses a slight problem for Microsoft: consumers get impatient and the hype starts to die away. Unlike Vista, and like XP, however, Windows 7 is out in time for the holiday season. Somehow, we think shoppers will repay Microsoft for this timing.

Keep an eye out for our comprehensive review of Windows 7 in the days ahead.