DeLi Linux - I use this myself. It an Operating System in about 50mb (750mb for a full install). Its great for older computers or use it on a newer computer and run it inside of Windows (not sure if it works in Vista).

Description from manufacturers site:
The trick is, that DeLi Linux uses only "lightweight" alternative software. If you are looking for the newest KDE, GNOME or Mozilla, DeLi Linux will not make you happy. The test computer is a PII laptop with 64 MB RAM, and all apps which comes with DeLi Linux are running smoothly.