There are lots of software products around which claim to provide a safe computing environment by preventing malware and viruses from ever accessing your hard disk. If you like the idea of such a product, then here's a great one to try.
LPS, as it's called, stands for Lightweight Portable Security. It's actually produced by the US Department of Defense for use by military personnel, but the licence explicitly states that it's also for use by the public.
LPS comes in the form of a CD image. Download it, burn it to disc, then reboot your computer. After a minute or so, you'll find yourself running a cut-down version of Linux. There's a familiar desktop, from which you can launch applications. Firefox is installed, and ready to use. If you opt for the deluxe version of LPS, which I'd definitely recommend, there's also OpenOffice. Again, it's fully installed and ready to run, so you have a word processor and spreadsheet at your disposal, as well as the rest of the OO suite.
But the most important part of LPS is not what's installed, but what's missing. There's no support for local hard disks. You can't access your hard drive from within LPS and, most importantly, neither can any malware or viruses. So, for example, if a visitor to your house wants to borrow your PC for some web surfing, boot them into LPS and there's no way that their activity can cause any harm. And there's no way for them to see what's on your PC either.
Because LPS runs entirely from the CD and memory, it's very fast. And there's nothing to install or configure - just boot the CD and you're running LPS. Reboot (there's a Start button in LPS from which you can do so) and, assuming you remembered to remove the CD from the drive, you're back in Windows.
LPS also has other uses. Because it's so lightweight and fast, it's handy for bringing an old laptop back to life if it's not quite powerful enough to run Windows.
LPS is a 137 MB download, or the deluxe version (with OpenOffice and Adobe Reader) is 317 MB. You won't be able to store documents on your hard disk, so you'll either need a USB stick and/or an account on something like Google Docs. And of course you'll need a blank CD onto which to burn the file. Which, with Windows 7, you can do easily. Just right-click the .ISO file once it's downloaded, and choose the "burn" option.
You can download LPS from http://spi.dod.mil/lipose.htm
If you want to make a bootable LPS system on a USB pen drive, see http://spi.dod.mil/liposeUSB.htm for details.