Need I say more?
Great for removing malicious software from your computer. Malware should not be consfused with a virus, though a lot of malware behaves similarliy to many computer viruses out there. It's free and awesome! Doesn't take much system resources and there is a paid for version if you're interested.
CD XP Burner
If you have anything older than Windows 7, then you can't burn ISO images from your computer. Bummer dude. If you need to burn an ISO image, then this is probably one of the better options next to upgrading to Windows 7. :)
If you don't have the money to purchase a full Antivirus Sweet, Microsoft Security Essentials is a good place to start. It's a light program (doesn't take up too many system resources) and it does a good job at removing what it finds. There are other free pieces of Antivirus out there, but they'll include pop-ups and some companies don't update their products often enough. Coupled with some malware removal tools you'll be set.
Screen Capture Software
Lets you take screenshots of your computer with one click.
That's all for now.