Vista - dwm.exe frequent crashes

I have a problem which i have been trying to solve for the last week it's
driving me mad.

PC is made by Maxdata:
MS-7173 board (with vista compatible bios)
Pentium D 3.4ghz dual core
2gb RAM (matched pair)
250gb SATA drive (samsung sp2504c)

additional hardware (tested with & without)
ASUS EN7600GS
Creative Audigy 4
Hauppage Nova-T DVB card

The problem is that DWM.exe keeps failing and restarting. Here's the event
log details:

"Faulting application Dwm.exe, version 6.0.6000.16386, time stamp
0x4549aed1, faulting module ntdll.dll, version 6.0.6000.16386, time stamp
0x4549bdc9, exception code 0x80000003, fault offset 0x00042ea8, process id
0xe14, application start time 0x01c771796f1cd577."

Whats strange is that these errors occur every 40 minutes, to the second.
The error occurs even when i am not using the PC at all.

This is happening with all additional hardware installed with latest
drivers, and also on a clean, updated install with no additional hardware or
software using only the supplied MS drivers. In order to troubleshoot I have
tried using both a PCI-E graphics card and also the on board ATI gfx chip, I
have also run the vista RAM test with no errors reported.

The fact that this is occuring exactly every 40 mins leads me to believe
that some kind of timed system process is at fault. Being only a half-geek i
am now stuck. Any ideas folks?