I usually set the boot partition for XP at 40 gig on anything from 80 gig upwards.
20 gig I would say would be the absolute minumum.
12 gig was ok for 98 and earlier but once you have a few programs installed on XP you will soon be approaching, if not already in, double figures.
If you havent moved your Swap file (Page File) to another drive then you will seriously harm the potential performance of your pc.
60 gig is the recommended size for Vista by the way.