Blue screen problems can be an absolute nightmare to diagnose. I had a similar recurring problem which I couldn't solve myself. An experienced (and expensive!) computer technician tried for me. After various software fixes he started replacing components. I ended up with a different motherboard, processor, video card and sound card! But the problem remained.
I eventually contacted one of the columnists for a national computer magazine. I told him that the problem had originally occurred after I'd had a (secondhand) CD drive installed. He couldn't see how it could be related to the problem but he advised me to replace it with a DVD re-writer. That didn't fix the problem either. But several disk formats and Windows re-installations later, I spotted that the blue screen problem only seemed to occur after I'd loaded the CD/DVD writer software. I eventually worked out that it was a software problem, related to a 'packet writing' program, in the burning suite, that I never even used.
All of that took over two years to get right (even with the help of an experienced technician and a nationally-respected computer journalist) so you might have a long job ahead of you.
Here's what I suggest for now:
Uninstall any software you've loaded in the months immediately prior to the problem first arising. If possible, also remove any new hardware. If that doesn't work, back up everything possible and try a 'repair installation' of XP:
http://michaelstevenstech.com/XPrepairinstall. htm
Chris