My system BSODs with the following entry in Event Viewer each time it happens. I leave the system on overnight and every day when I return I get this BSOD.

The computer has rebooted from a bugcheck. The bugcheck was: 0x1000000a (0x000000c4, 0x0000001c, 0x00000000, 0x805023d7). A dump was saved in: C:\WINDOWS\Minidump\Mini071206-01.dmp.

The BSOD is IRQL_NOT_LESS_OR_EQUAL
STOP 0x0000000A 0x00000004 0x0000001C 0x00000001 (0x8050242F)

I have checked that the fans are clean of dust and overheating does not seem to be a problem. The memory also passes memtest.

My minidumps are attached.

All of your minidumps are caused by win32k.sys . Most of what I have read refers to possibly faulty ram. Faulty ram can pass some of those memory tests. If you have more than one module installed try taking out 1 at a time and see if the crashes occur. If that doesn't fix the problem, uninstall and reinstall Windows SP2. If that doesn't work try running the system file checker and see if that finds the problem. Click Start, run and type sfc.exe /scannow

For what its worth, i have to agree with fastco. IRQL_NOT_LESS_OR_EQUAL
pretty much are ram issues.
Again, you have to test one stick of ram at a time and let the test run for at least 6 passes for each stick of ram.
Another way to test is to just boot up with one stick of ram and then swap out for another stick and see if the pc works ok with one stick but not the other.