Regarding your hijackthis log: I would investigate the process called C:\WINDOWS\explorer222.exe
Normally, you would have a program called explorer.exe in that folder. Check if that one is there. If it is, compare the two (binary, checksum). There could be good reason for the explorer222.exe program to be there, but at least you should try to find out why it is there and how it got there.
There is a newer version of AVG available. I have had a virus on a Win 2000 server about 6 months ago. AVG didn't recognise it, presumably because the virus had only been in the wild a couple of days.
The virus replaced the regular explorer.exe shell with its own. It then proceeded to run a couple of window-less internet explorers every other minute, evidently to bump up some websites visitors statistics. It was also logging keyboard keystrokes, apparently it was looking for usernames/passwords for some mainstream MMORPGs.
It created itself multiple startup points, in all the well known locations, and in some less known locations as well. Everytime the machine was restarted it created several copies of itself with random names and set itself up to start at least one of them at the next restart.
It was discovered because the machine would randomly slow down or spontaneously restart.
Cleaning the virus out of the machine was a big bad pain in the arse job that took almost all day. Don't ask about the rest of the network...
Anyway, I don't want to press the panic button ( I guess I made a good job of doing just that
), just trying to illustrate that there could be a number of different causes for your problem.
So why don't you open your box, start it up and check that the CPU and graphics card cooling fans are in good working order. And while you're at it, take a brush to them and clean out all the accumulated dust from the fins of the fans and the heat sinks (switch off and unplug first). That shouldn't take more than 10 mins and may just solve your problem.