We did that and it took a long time to get the little things (internet connection, re-installing printer etc drivers) working again! I don't really care what operating system I use, so XP doesn't seem that whizz-bang to me, just takes up a lot more hard disk space as you mentioned.

Also, 128mb of RAM wasn't enough any more, and we had to go to 256mb of RAM just to get XP going at a reasonable speed. Good lesson for us on how Win-Doze got its name