This happens sometimes for hours at a time it just keeps rebooting and doing a disk scan. It will then start normally and I can work sometimes for hours and sometimes it will immediately start the reboot process all over again.

I have scanned for viruses, ad-ware and worms with Nortan antivirus, SpyBot, and now have Windows Live One care installed. I have not found any strange programs. I have tried the Windows XP upgrade disk to repair but my upgrade disk apparently has no repair option. I have also tried a system restore to a pint before this all started but this did not help either. In startup & recovery I have disabled automatically restart. The hard drive appears to be working continualy if the little ligtht in the front of the computer is any indication.

I have installed an external hard drive and backed up my critical files to it however the backup software does not do full disk backup.

The following is my system information minus the system name for security reasons. Any help would be greatly appreciated.

OS Name     Microsoft Windows XP Home Edition
Version     5.1.2600 Service Pack 2 Build 2600
OS Manufacturer     Microsoft Corporation
System Name     
System Manufacturer     Compaq
System Model     Presario 5150WM 470019-783
System Type     X86-based PC
Processor     x86 Family 6 Model 4 Stepping 2 AuthenticAMD ~1302 Mhz
BIOS Version/Date     Compaq 786K3, 7/26/2001
SMBIOS Version     2.3
Windows Directory     C:\WINDOWS
System Directory     C:\WINDOWS\system32
Boot Device     \Device\HarddiskVolume1
Locale     United States
Hardware Abstraction Layer     Version = "5.1.2600.2180 (xpsp_sp2_rtm.040803-215"
User Name     MARAJF\default
Time Zone     Eastern Daylight Time
Total Physical Memory     256.00 MB
Available Physical Memory     19.29 MB
Total Virtual Memory     2.00 GB
Available Virtual Memory     1.96 GB
Page File Space     605.96 MB
Page File     C:\pagefile.sys

   It sounds like a bad power supply to me.  A bad power supply will just shut the computer off and then it will go through the normal startup process (since it was shut off abruptly, a scan is done).
   The easiest way to check for a bad power supply is to replace it and see if that solves the problem.  The power supply is a part of your computer (the box that has a fan in it on the back of your computer) and costs around $50.  They do come in different sizes so make sure the one you buy will work on your computer (ask the people at the parts store).  Too big is usually OK, but too small is not.