...the CMOS battery was dead because it wasn't correctly saving the settings. It's also possible the power supply is going bad.
What happens when you hold the power button in for 10-15 seconds? Does the computer then shut down entirely? Or does it restart again after it has shut down?
I would start by replacing the CMOS battery which is only about $5 and can be done easily by you.
It also appears like the XP operating system may have been an "upgrade" from a previous operating system. Is that the case here? If so, you may need to upgrade the BIOS? Have you ever flashed the BIOS with the upgraded version for XP?
Hope this helps.
Yester day I turned my PC off the way I always do. Start,Turn off, Turnoff. But now it restarts when I Turnoff. Help
OS Name Microsoft Windows XP Home Edition
Version 5.1.2600 Service Pack 2 Build 2600
Processor AuthenticAMD ~1000 Mhz
BIOS Version/Date American Megatrends Inc. 62710, 8/6/2002
SMBIOS Version 2.3
Time Zone Pacific Standard Time
Total Physical Memory 512.00 MB
Available Physical Memory 291.05 MB
Total Virtual Memory 2.00 GB
Available Virtual Memory 1.96 GB