Verify your bios settings are the same as they were.
Strangely enough, when i upgraded to Vista, i was getting random crashes and stuff. After a few weeks of this, i ran orthos and it errored almost immediately. Loosened my ram timings and it improved, but still errored. I thought it was Vista.
Went back to XP (dual boot) and same results as in Vista, Orthos errored out. As it turns out, my ram Voltage was somehow set back to stock 1.8V. I needed 2.1V to get it stable.
I have no idea how installing Vista could reset my ram voltage (no way in hell it should be able to), and it may have just been a coincidence with the install. It's worth a look though.