Two things came to mind:
As nemesis, the accuracy of the readings from HWMonitor (bad thermocouple? - The BIOS suggests not.) Crashing from the BIOS? What year and make is the machine? Does it have an EFI or traditional BIOS?
Second thing that came to mind is I seem to recall reading about the potential emergence of hardware thermal protection exploits in malware. Run RootKitRevealer, study the results carefully. Do you happen to have a virtual sound device driver installed that you never installed yourself? (A very versatile and powerful rootkit I've been seeing around lately....)
Are these memory sticks original to the machine or new? If new, who installed them?
What is an EFI?
system specs , a downgraded Gamer Ultra 7500 SE purchased from CyberPowerPC in March 2007:
CoolerMaster Stacker 830 Tower W/ Side-Panel Window
(Socket AM2) AMD Athlon™64 X2 3600+ Dual-Core CPU w/ HyperTransport Technology
FAN: AMD ATHLON64 CERTIFIED CPU FAN & HEATSINK + 3 EXTRA CASE FANS
HDD: Single Hard Drive (250GB SATA-II 3.0Gb/s 8MB Cache 7200RPM HDD)
MOTHERBOARD: (Socket AM2)GigaByte GA-M55SLI-S4 nForce4 SLI Chipset DDR2/800 SATA 16x PCI-Express MBoard w/GbLAN,IEEE1394,USB2.0,&7.1Audio
MEMORY: (Req.DDR2 MainBoard)1GB (2x512MB) PC6400 DDR2/800 Dual Channel Memory
XP Home Edition w/ Service Pack 3
POWERSUPPLY: Apevia 680W BEAST POWER ATX Power Supply
SOUND: HIGH DEFINITION ON-BOARD 7.1 AUDIO
VIDEO: NVIDIA GeForce 9600 GT 512MB PCI Express x16
16X DVD ROM
Tried RootKitRevealer but had to abort 10 minutes in as scanning pics in the My Documents folder had the temps up to 226F on Core#1 and 240F on TMPIN2.