super-high speed fan, system freeze

Hi, over the last couple of days my computer has been having trouble starting up and shutting down. It seems to be worse after it's been on for a few hours (4-5). I opened up the case to check for dust, very little on the tops of the fanblades and heatsink (I dusted this machine about 2 months ago). Upon first startup today the cpu temp was 38°c, after about 6 hours of mostly being online chatting and surfing, the temp has been steady 42°-44°c (according to Speedfan).

An app that I tried before I downloaded Speedfan called Fresh Diagnose said I didn't have any fans connected. When I clicked on the Sensors tab agian the CPU fan instantly went into super-high-speed, and according to Fresh Diagnose the CPU temp. shot to 110°c, and CPU usage shot to 100% and the machine froze, I had to hold the power button in to shut down. Every time I clicked the Sensors tab in Fresh Diagnose this happened. I uninstalled it after it done this 3 times.
Here's what it is doing:
1.) Every time I restart or shut down the machine from the start menu/shut down (when it will) the fan kicks into super-high-speed (sounds like a turbine on a jet) for about 3-4 seconds before the computer actually turns off.
2.) When it starts, everything appears to load; except the apps that start when windows starts (the ones that are on the taskbar beside of the clock on the right). Which means nothing is really loading I suppose because I can't click on anything; the start button or the desktop icons. I have to hold the power button in.
3.) When shutting down it appears to go through the process normally (says windows is shutting down etc. but hangs. I have to hold the power button in.
It starts fine in safe mode, AVG scan shows nothing. I haven't ran HJT since this started.

Presario SR1103WM
WinXP Home sp2