I convinced all my friends to upgrade to Windows XP because it's so easy to use and not very demanding in terms of hardware. I believe the weakest system I put it on was a Pentium 2 300MHz, 64MB SD RAM and 1GB harddrive. I had to use nLite to cut down windows XP's size so it would fit, but it worked flawlessly once the drivers were loaded. I think Windows XP was at 540ish MB after installation, took quite a while to find the drivers though having had to cut them all out of WinXP, yet it retained surprising functionality.
Correction: I remember taking the machine apart and finding an old-school pentium MMX, at least thats what it said on the processor