If you use Windows 7 or Windows 8.1 and want to upgrade to Windows 10 for free, there are just a few more hours left to grab your copy. The offer expires today, July 29. At the time of writing, less than 19 hours still remain.

If you're eligible, you should upgrade. In almost every regard, Windows 10 is a better operating system than Windows 7 or 8.1 (unless you use Media Center, in which case you're trapped on old operating systems forever). If you want to get the free upgrade but can't upgrade right now because of timing or compatibility concerns, your best option is to install Windows 10 onto an empty hard drive using your existing Windows 7 or 8.1 key. Activate that installation and magic will occur: your key will become Windows 10 "enabled," and you should be able to use it to perform the upgrade at a later date. Similar results can likely be achieved by installing into a virtual machine rather than an empty hard disk.

The cut-off doesn't apply to those who use assistive technology such as screen-readers; those Windows users will be able to upgrade to Windows 10 whenever they feel like it, though Microsoft apparently has yet to fully explain how this will work.