Just got Ubuntu 11.04 installed in an old Compaq (1.8ghz sempron, 512MB Ram, you get the idea!). Was easily able to install the flash plug-in for Firefox and that was all I could see as a necessity! It just works, was able to test skype, facebook games (bejeweled), and videos on facebook and youtube. It comes built in with open-source antivirus and can be set to auto-update itself. Set it and forget it! Mac people brag about not getting viruses, but nothing beats Linux for security.
http://www.ubuntu.com/
If this is something you are interested in trying as a second option to Windows or to replace it, please contact me!
-Mr. Ricke