It can also be hardware.
Just two weeks ago I was getting the BSOD with that error message and I spent a long time messing around with virus checkers and eventually reinstalling Windows. This system had been running stable for more than 6 months prior to that.
Anyway, the result was that nothing worked. In the end it turned out to be a bad DIMM (found by process of elimination, since I had 4 identical 1gb DIMMS). To compound matters, that bad DIMM caused a strange error message when I attempted a repair install of Windows. I got the message 'NTOSkernel.exe' is missing. Please replace the file' or something similar. I attempted to copy the file manually from the command prompt and the copy failed, saying source file not found, even though I could see it when listing the contents of that DIR on the CD.
Ditched that DIMM and the problem was solved.
Put the DIMM back for giggles, and the problem came back. Moved the DIMM to a different slot and the problem only came back after loading several large files in Photoshop.
Just for reference sake, these are the error messages I got with a known bad DIMM:
1) IRQ_LESS_THAN_OR_NOT_EQUAL
2) MACHINE CHECK EXEPTION
3) PAGE FAULT IN NON-PAGED AREA
4) NTOSKernel.exe file missing, please replace (on reinstall)
Quite frankly, I haven't found these error messages to be all that helpful. On my old machine I got number (3) often, and for various reasons, but not because of RAM.
