|EDIT: Just saw your post, which renders most of my advice useless...lol|
Here are some far out ideas:
-Did you accidentally bash your hand against any component when you were hooking up the case fan? Particularly the CPU cooler or any expansion card? If you did, you might wanna reseat the cooler or expansion card. Poorly seated cards can cause a computer to behave strangely.
-Do you have a card reader? If so, try unplugging the reader from the motherboard. I once had an internal card reader that completely clogged up my system. The reader was fine for several months, but then one day when I fired up the machine, Windows took a good five minutes to reach the welcome screen and another five to get to the desktop. It took me forever and a day to find out that the card reader was the culprit. After finally unplugging the reader from the board, Windows booted in a flash. Nearly bald from all of the troubleshooting, I went to the store, bought a new reader, and haven't had any problems since.
-Hard drives and optical drives--are they in DMA mode? This is usually not a problem with SATA drives, but worth a check.
-Memtest your memory. http://www.memtest86.com/
-In the BIOS: CPU cache enabled? Is all memory recognized and at the right speed?
-All internal power cables plugged in?
Socket 939|Dual-Core Opteron 185 @ 3.2GHz|A8N32-SLI Deluxe|4GB CL2 PC3200|SLI 8800GTS|1TB|PCIe X-Fi Titanium Pro|Win7 Ultimate x64