During startup XP reboots after windows XP home appears

Hi,

I have a Pent III computer with 512 Mb of RAm and a 250 GB hard disk. I installed or upgraded to XP about 6 weeks ago. The computer was working fine. Now when I turn it on it starts to bootup but right after the WIndows XP home edition window appears, I hear a click and then it reboots. It continually does this unless I turn it off. One note to mention, the first time it displayed the windows XP home edition window it automatically went into chk dsk. It found no errors then the system rebooted and started this boot cycle. The system no longer goes into chk dsk, it just reboots.

I tried booting in safe mode but it doesn't work either. i also tried booting in "last working mode". No luck.

I checked the CPU temp and it said 52 deg C. Is this OK? Tha fan is working at 4582 RPM. The voltages looked reasonable compared to the listed specs. The only one I am not sure about is Vcore=1.66V .

I have Nortin anti-Virus on my compter and it is updated weekly. I also have Adaware and run it weekly as well.

At this point I don't know what to do. Any advice is appreciated.

Thanks,

Ernie

PS. One more thing to mention. When I upgraded to XP from ME, the one problem that occurs is that the system will not shut off automatically. The XP windows display says ready to shutoff. I can only shutoff by hitting reset and then the off button. Microsoft said I probably have to update the BIOS to fix this. I have AWard Modular BIOS v4.51PG.