I'd be more concerned over exactly where the space is going rather than 'fixing' it by giving it more with changing partition sizes. My first question would be what else do you have on that Windows partition? Space doesn't just vanish into the ether - something is filling it up... and as lousy as Windows updates can be, if you've given XP a reasonable amount of breathing room on its partition (15-20GB) Windows Updates shouldn't have chewed it all up.
How big is that Windows partition?
Have you got lots and lots of iTunes music on it? ALAC in particular will chew HDD space like it was going out of fashion. This is my main suspect in terms of HDD space vanishing...
Have you put any more RAM in the system? If you've got the Hibernation and Page files active and Page file is set to size control by Windows (the default) then putting more RAM in will make the Hibernate and Page files jump in size...
Are there any large .log or temp files hiding anywhere?
Do you have System Restore set to a large percentage of the partition?
Are there lots of "$ folders" in the \Windows\ directory? (They'll be hidden and compressed with NTFS compression - they're often left behind when Windows updates essential system files, as they're the cache from the install and the old version in case you want to uninstall the Windows Update... but if you aren't going to uninstall the Update, they're just chewing space)
May also be worth downloading something like CCleaner and giving it a run to see what it finds; it's usually pretty good at finding temporary files hiding in daft places.