Sometimes we all do things on our PC which we know we shouldn't. Such as browsing P2P systems, or checking out web sites which are of interest but which seem slightly suspect. Or allowing a friend or relative to use the computer, and having no idea what they're doing with it.
Traditional security software such as anti-virus and anti-spyware can obviously help in such cases, but a better solution would be to use virtualization or sandbox technology to creat what is, in effect, a totally separate computer in which you can safely conduct your experiments or your friend can surf freely. Products such as VM Player or VirtualBox can help here, and they're both available free of charge (though you'll need licences for any operating systems that you install onto them).
Another option is to use a program such as BufferZone, from Trustware (www.trustware.com). It uses virtualisation technology to create a safe web-surfing environment, in which any attempt to modify the contents of your computer is merely simulated and doesn't actually have any lasting effects. If you inadvertently download a virus, for example, and it attempts to modify the registry or copy itself to your hard disk, the virus will think that those operations have succeeded whereas, in fact, no registry amendments have been made and no files have been created.
BuffzerZone is available as both a free version and also as BufferZone Pro. The "Pro" version has additional features and normally costs $39.95 for a one-year licence. However, until the end of 2010, Trustware is giving away the software for free. So head over to www.trustware.com and get a year's worth of protection for nothing. It's a 15 MB download, and runs on Windows XP and above. Free support is offered in their online forum.