Dell Dimension 8400 blue screen drives me crazy

Greetings!

I have a friend's Dell Dimension 8400 on my desk throwing blue screens.
I have gone through a lot of steps (see below) to try and fix this to no
avail. I therefore need your help!

Symptoms:
- BSOD with PAGE_FAULT_IN_NONPAGED_AREA
STOP: 0x00000050 (0xFF99FFF8, 0x00000000, 0x80551211, 0x00000000)
Stop error parameters are identical every time
- No additional information is displayed about the component that
causes the BSOD
- Minidumps are enabled but not created

When does the problem happen:
- While running the Microsoft Malicious Software Removal Tool
(MRT.EXE). This happens
- when running the MSRT mannually
- when trying to install Internet Explorer 8 (this runs MRT.EXE)
Note: I managed to get IE8 installed by killing MRT.EXE manually
before the system crashes
- during automatic updates (which MRT.EXE runs every month)

System information:
- Dell Dimension 8400
- BIOS A09 (latest)
- Windows XP Home Edition SP3, fully patched
- 2GB Memory
- 80 GB primary SATA disk (WD Caviar WD800JD)
- 250 GB secondary SATA disk (....)
- ATI Radeon X300 display adapter

What I have tried so far:
- check for bad RAM with Memtest86: no errors after 9 passes
- replace RAM
- update all drivers (including iastor.sys and ATI display
drivers) to latest versions
- reinstall SP3
- uninstall patch KB977165 (MS10-015)
- remove end-of-life software (such as Nero 6.6)
- run full chkdsk.exe on all drives (no errors)
- check the system drive for viruses and other malware by
attaching the system drive to another system (so that root
kits cannot hide) using various tools
- clean the registry

So far all attempts have failed and I have run out of clues.

== Any pointers? Please help!!

Best regards & thanks,
-Roger