Here's the histoy to the problems I've had tonight.
System froze when trying to download quake 4 demo off id site. Thought it might be a problem related to Steam on my ID (seems to be slowing my boot time lately), so I rebooted and logged on to my Dad's ID and downloaded it from there. I also set Steam not to load on boot from my logon.
Got Q4 working and played through the demo before trying multiplayer. Halfway through mulitplayer I lose sound. Quit the match and try to go back in, but screen goes black before getting to the match, forcing a reset-button restart.
Now I get BIOS errors saying "keyboard error or keyboard not present". I use a USB keyboard plugged into the back of the case (mobo backplane). I eventually got the system to boot with the keyboard plugged into the front USB port of the case, whereupon it said that the integrity of one or more disks needed to be checked. I skipped this and got to the Windows logon where I shut the system down.
Any ideas as to what happened? When I had the case open I made sure each cable was fully pushed home (SATA cables are the old, non-locking type). I'm not reliant on this system (sending this from laptop), but it's a very expensive paperweight otherwise!
Theory: I've been having heat problems of late, could this have temporarily sent the mobo a bit screwy?