I have a 20 gb and 5 gb hard drive. I put this pc togather for my parents to play on the internet. Is there an advantage to installing win XP of one or the other?

You are always better off putting Windows on the Fastest,newest,most reliable, drive you have ,so if these two drives are close in terms of what I just stated then make the 5Gig the active primary,the 20 the slave .
The 5Gig is just big enough for Windows, especially if for email and surfing , I would suggest making the 20 Gig for data,Temp files etc to minimize the amount of space used by extraneous programs on the smaller drive.