Looking over your post I see you omit details as to why you only run Windows XP (no service pack.) The machine details are missing as well as the story about it working last week or never.
Im running a pc with windows xp.
Every time I run ANY type of flash my computer slows then reboots on its own after a few minutes.
I have reinstalled flash a few times. No change except for a while, it takes a little longer before it reboots.
I see this problem with flash applications, online flash games, youtube, etc...
I wanted to see if there was an easy fix before I wipe my hard drive and start over.
Also, just in case its related, anytime I right click on anything on my desktop or anything in a folder my computer freezes to the point where all I can do is reboot.
If its not related I will work on it another time.