I recently downgraded from Vista Ultimate 64-bit to XP Pro 64-bit and I've noticed my system doing 1 strange thing from time to time:
When I boot up my computer it gets to the Windows XP loading screen where it fades in, but it doesn't fade in completely and my system stalls there.
Before anyone points fingers at my OS, it used to happen when I had XP Pro 32-bit too. Never had any such problems with Vista, so I don't believe it's a hardware issue.
All drivers are latest, all Windows Updates are installed, and I am completely out of ideas.
CPU: Q6600 @ 3.6 GHz (tried at stock speed, still stalled)
Mobo: Asus P5KC
RAM: 8gb OCZ Platinum @ 800 MHz, CL3 (tried at stock of 800 MHz CL5, and tried with 4gb, still stalled)
HDDs: 1x WD Raptor 150gb, 2x WD Caviar 500gb (original XP install was on the Raptor, current is on a Caviar, so not HDD related)
GFX: Gainward 8600GTS Gold Edition (normally overclocked slightly, stock currently)
PSU: Point of View Black Diamond 500W (tried with a 750W Thermaltake Toughpower)
Audio: Razer Barracuda AC-1 (new addition with this install, used the on-board before)
All unneeded ports on the motherboard are bios disabled (firewire, on-board and front audio)
I hadn't tried it because I wanted to keep 2 sticks of ram in for dual-channel-ness, tried 1 stick and after a couple of restarts I got the same issue (as it doesn't happen every time it normally takes a few restarts to force it to happen, though most of the time when I cold boot after a few hours it stalls)