An event ID 51 entry is a generic error entry for any type of error that occurs when Windows is paging information to input/output (I/O). A paging operation occurs when Windows either swaps a page of memory from memory to disk, or when Windows retrieves a page of memory from disk to memory. This is part of the memory management features of Windows XP.

Unfortunately, your error message can well mean more than one thing:

-It can be caused by your Power Supply being either faulty or not powerfull enough. The only way to test this, is to put in a new, more powerful one.

-RAM can sometimes upset the system and produce similar errors to those you have.
The best RAM check is to replace it, one module at a time, with known good RAM.
You could go here http://www.memtest86.com/ and get this software to test your RAM.

It might be the case that your drive on the Primary IDE channel or Secondary IDE Channel is running in PIO mode instead of DMA Enabled mode. To check this go into System->Hardware->Device manager->Double click IDE ATA/ATAPI controllers->Double click Primary IDE Channel/Secondary IDE Channel->Advanced Settings->Device 0, Transfer Mode should be set to DMA if available, Current Transfer Mode set to Ultra DMA Mode 5 or 6.
If this is set in PIO mode you need to click on the Driver tab, select Uninstall, click ok, the Primary/Secondary IDE Channel will disappear, then re-boot, this will then re-install the Primary/Secondary IDE Channel. Then go back into Device manager and set it too run in DMA mode.
What can happen on occassions, if there are more than 6 errors that occur on a Device, it stop running in DMA mode and runs in PIO mode instead