If anyone has a solution to this I'd be very grateful.

I have a self-built HTPC (specs below) which has been a nightmare from day one, most likely as I was a cheapskate and used a combination of old bits I had and cheap rubbish from ASUS.

ASUS A8V-MX motherboard
AMD 939 X64 3800+ CPU
1Gb 3200 memory
Hauppauge HVR-3000 PCI TV tuner
Sapphire HD 2600 XT graphics card (AGP)
Windows Vista Home Premium

Okay, scene set. My problem is that whenever I try to install the HVR-3000, the system hangs instantly as the drivers are loaded. Without drivers the computer will putter along with the card in it fine. If I try another PCI card (eg a gigabit ethernet card I have) that is exactly the same (and I've tried three so far), so it really seems to be the PCI bus on the motherboard.

I have tried a fresh install of Vista, unpatched, with the WinTV 4.4 drivers: crash
I have tried a fully patched copy of Vista, same drivers: crash
I have tried a fresh install with all other drivers loaded first: crash
Fresh install with no other drivers: crash

Since the machine dies instantly without writing any logs, event viewer is useless. Once the crash has occured the only option is to reset/turn off the PC.

Weird thing is... I did get it going briefly after a bios flash, but further fiddling to try and get HD acceleration on the 2600 going killed it again. If I go in on safe mode I can load the drivers fine, but then the machine hangs part way through booting afterwards. Someone pointed me towards the ASUS ATK0110 thing on the motherboard, but I can't find a way to disable it. Failing to load drivers for it on fresh install doesn't help either.

I don't have the cash to bin this heap at the moment, so I'd really appreciate any experience with these components anyone may have. Please tell me I'm not the only person who bought these particular bits? :)