Just some educated ideas. If you only have the one comp, go to a friend's house to use his/hers. You have to tell us exactly when the BSOD occurs. I'm making the assumption it's before you reach the XP logo screen.
1. Unless there really is a problem, one should not upgrade the BIOS. But you flashing it, would not cause the BSOD's five days later. If the flashing would have been the problem, you'd have seen it right away.
2. You didn't mention hd's size, age, or flavor, but many of them start developing bad spots. The only utility I know of to help you out of that problem is SpinRite v6.0 from grc.com (about $89). Run it level 4. Depending on size, this could take 18-26 hours.
3. Memory, sometimes, just goes bad. Get MemTest (free) - make its' cd, and run it for at least 4 hours. Every half hour or so, check the working screen for errors. If you have two sticks, remove bank 1 and run the test. Then, remove the bank 0 piece, pop in the one you removed earlier into bank 0, and run the test again.
4. Power supplies can also go bad. When they're not supplying the proper voltages (or wattage), your hardware will not function right. I suggest purchasing a compatible (at least the same wattage - more would be better; and physical size) power supply, and swap out. If this results in having a spare, this is a good thing. Every one owning a pc, should have a spare p/s. It's one of the first pieces of hardware that goes bad. ($40-$50)
Because what you describe is almost always hardware related, you have to eliminate those three areas I talked about, as the problem, first. It'll take you a couple of days to accomplish that. If they all come up clean, this is a good thing for you to be armed with knowledgewise; and a relief.
Then you're faced with these:
1. Boot sector virus (mbr corruption)
2. Corrupt boot loader files (also possibly caused by a virus)
3. Corrupt O/S files - dll's, drivers
4. You may have to reinstall XP if the various tries at fixing don't work.
When you pop in the XP cd, does it run as if it is doing an install?