A laggy old PIII was my test subject yesterday. Installed xubuntu desktop v. 8.04 (a linux distribution) on it and it was browsing the internet as fast as my desktop which I am continually tweaking. It was unbelieveable. Just wiping Windows turned a reject computer into something I'd want to take home with me! Incredible.
Xubuntu is available at http://www.ubuntu.com free of charge. The desktop version is 544 mb for x86 computers. The iso needs to be put on a cd. The install is a breeze.
Choose guided instead of manual partitioning and the swap partition will be made for you.
Kicking myself for thinking crappy old computer when indeed it was crappy old Windows.