Out of the blue last week I got the blue screen of death. Restarted the computer and it took an absolute age to reload, but seemed ok ish after that. Ever since then, every time I've started up my computer I've been prompted to register it (I did that when I bought it).
That sounds like corruption of the filesystem. Do you have the XP installation CD? If so, reboot from that (you may have to reset the BIOS to boot from DVD/CD) and try using the recovery console to fix any errors. A corrupt filesystem
could explain system instability but I've never come across anything like that before myself.