What size partitions would work best

Heres what i usually do:

50 go for windows
100 for programs
rest for data (350 go)

I install small programs like anti-virus adobe reader, antispy with windows part
Bigger programs like office, webexpert, games on program partition (sise could be bigger if have lots of games and big programs

(to be safe you could do 100 /100 / 300)

hope it gives you insight

i have xp home sp 3 fully updated . i use a 10 gb partition for xp it takes up about 4.25 gigs . i have other apps and programs that are on other partitions . they also and a folder to the xp partition sometimes but they dont take up much room .

You can format your OS partition.. install your nicely-working fresh OS, and keep both data and programs.

Yes, so the benefits of having data on a different one are obvious. About the programs.. you are somehow right, but think that you could re-install the apps on the same places (to create these OS files and registry keys), and preserve all the program data you had, which is also some kind of benefit, from my point of view. Despite there are many programs that work without those "OS files&registry keys", you just exec them.. those would also remain untouched.

If you use an imaging program after you install your apps it will image every thing on the C drive including your start - programs menu - so your apps links will still work . You dont have to reinstall all your apps . Even if they are on other patitions . You just wipe the C drive - format and use your imaging app to reimage . Everything should work like before .