I have not installed 2k and XP together but I've installed Win98SE + XP or ME + XP on several computers, with the two OS'es together in C:.
In most cases, they share the same default location for Program Files in C: No problem installing the same program for each OS in the same directory. The only exception I've come across so far is AVG. For AVG, installing for one OS always corrupts a previous installation for the other OS.
I thought I'd try installing AVG in different directories for the two OS'es, but have not actually done so yet.