I saw a lot of that when I did tech support... the five main causes were a bad power supply (most common), system memory, faulty memory on a graphics card, a hot processor on graphics card (especially when using graphic intense programs) or mbr (not too common)
Restoring the MBR - These work, but like anything else with XP, you run the real risk of screwing up, not being able to access volumes and loosing everything... always back up your important files before screwing w/ XP in any way
Restoring the MBR - FOR THOSE THAT AREN'T COMPUTER CHALLENGED
You must repair the MBR if it becomes corrupted and you can no longer access any volumes on that disk. You can use several tools to repair the MBR. Which tool you choose depends on whether the partition table is also damaged and whether you can start Windows XP Professional.
Use the Recovery Console. You can use the fixmbr command in Recovery Console to repair the MBR. You can start Recovery Console by booting from the Windows XP Professional operating system CD; so this troubleshooting method is available even if Windows XP Professional does not start in normal or safe mode. However, you cannot use Recovery Console to repair partition tables that were damaged by viruses or other corruption.
Use DiskProbe. You can use DiskProbe to restore both the MBR and the partition table, but you must have previously backed up this information by using DiskProbe, and you must be able to start Windows XP Professional.
Use a third-party disk editor. You can use a third-party MS-DOS-based, low-level disk editor to repair the partition table if Windows XP Professional does not start. This method is for experienced users only and involves manually editing the partition table.
Using the Recovery Console to Replace the MBR
You can use the fixmbr command in Recovery Console to rewrite the MBR to resolve a corrupted MBR on a startup disk. However, running fixmbr overwrites only the master boot code, leaving the existing partition table intact. If the corruption in the MBR affects the partition table, running fixmbr might not resolve the problem.
Caution
Use this command with care because it can damage your partition table if any of the following apply:
A virus is present and a third-party operating system is installed on the same computer.
A nonstandard MBR is installed by a third-party disk utility.
A hardware problem exists.
It is recommended that you run antivirus software before you use the fixmbr command.
To start the computer and use the Recovery Console to replace the MBR
Insert the Windows XP Professional Setup CD-ROM into the CD-ROM drive.
Restart the computer. If prompted to press a key to start the computer from the CD-ROM, press the appropriate key.
When the text-based part of Setup begins, follow the prompts. Press the R key to repair a Windows XP Professional installation.
If you are repairing a system that has more than one operating system installed, from the Recovery Console choose the Windows XP Professional installation that you need to repair.
Note
If you press ENTER without typing a number, the Recovery Console quits and restarts the computer.
The Recovery Console might also show valid installations of Windows NT 4.0. However, the results of attempting to access a Windows NT 4.0 installation can be unpredictable.
When prompted, type the Administrator password. If you do not have the correct password, or if the security database for the installation of Windows XP Professional you are attempting to access is corrupted, Recovery Console does not allow access to the local disks and you cannot repair the MBR.
To replace the MBR, at the Recovery Console command prompt, type:
fixmbr
Verify if you want to proceed. Depending upon the location and the cause of the corruption within the damaged MBR, this operation can cause the data on the hard disk to become inaccessible. Press the Y key to proceed, or press the N key to cancel.
Using DiskProbe to Replace the MBR and Partition Table
If you have backed up the MBR by using DiskProbe, you can use it to restore the MBR on any disk that is not used to start the computer. Restoring the backup MBR rewrites the entire sector, including the partition table. However, DiskProbe only runs under Windows XP Professional, Windows 2000 and Windows NT 4.0. It does not run under MS-DOS, Windows 95, Windows 98, or Windows Me.
If the disk that starts Windows XP Professional has a corrupted MBR, Windows XP Professional does not start. Therefore, you cannot use DiskProbe and must use the Recovery Console to replace the MBR.
For more information about restoring backed up MBRs by using DiskProbe, click Tools in Help and Support Center, and then click Windows Support Tools.
Using a Third-Party Disk Editor to Replace the Partition Table
Before you can repair the partition table, you must know the exact values to use to recreate the partition table. If you backed up your MBR and partition table by using DiskProbe, and you have the backup available on a floppy disk or on another computer, you can use DiskProbe on a different computer to see the correct values so that you can manually recreate the partition table.