It doesn't always happen... and, although I have added new hardware to it (just yesterday), it can happen any time (But it doesn't happen everytime I try booting winXP). Therefore, I don't think its to do with any new hardware I've added since I made it.
And no, I haven't tried booting in safe mode.
Furthur to the above post, It happened again today, and my technique of re-saving the BIOS didn't work...
In the end, I used a bootable windows CD (which worked), and then, supprisingly, my winXP installation worked OK again [it seems to have fixed it for the moment
] But I don't know how long it will last.
Soon, however, I will be changing winXP home for Pro, so i might just do a clean install