Depends on what options you install, have just rebuilt my XP workstation at home and as using it only for Audio have removed Internet explorer networking, messenger, games, desktops, animated menus and all that automatic error reportting crap etc and got it down to just over 1 Gb I think.

Theres a few good articles on http://www.musicxp.net/ about tuning XP for audio and reducing the installation size is kind of a by product of following their tuning guide.

Other things to bear in mind are...

If you switch of system resore and hibernation mode you reclaim significantly large chunks of disk space.

If your after a small XP install to go on a small hard disk just remember you need at least three times the size of your RAMs total capacity in free disk space to serve as a page file (virtual memory) XP doesnt take to kindly to you messing with these settings...