A friend's computer started having the STOP error about a month ago. She has 3 kids and a husband, and they all use the same comptuer. They download all kinds of stuff, and check email, and install many different programs on the computer. It is running Windows XP Pro and has Norton installed and updated on it.

The STOP error does not show up everytime I start the computer, but shows up intermittently (somtimes at startup, and sometimes after 2 hours of use, etc). I have ran antivirus, performed disk checking and defragmenting, gone through the registry and manually looked for inconsistencies (according to MSFT's knowledge base), and nothing solved the problem.

Here is the basic gist of the error:
IRQL_NOT_EQUAL_OR_LESS
*Disable Caching and Shadowing in CMOS*
STOP: 0x0000000A (0xf779fc54, 0x000000ff, 0x00000000, 0x804fdc2c)

I plan to get her a new hard drive and OS today, and transfer her files (after another virus check), so any fast responses and help would be VERY APPRECIATED!

It is very general error that you're getting, mostly related to memory or/and video card errors.

You can try installing original video card driver.
The message suggests that you disable Caching and shadowing in BIOS. You can try that too, if someone was messing with the BIOS.