Recurring BSODs

Quad core Q6600 @ 2.40GHz
2Gb DDR
GeForce 8500GT
Not sure of mobo
450V PSU
Various USB things (logi G5 mouse, Saitek Eclipse II kb, Sennheiser USB soundcard from PC-155s, M-Audio Oxygen 8v2)

There is a reason for posting what I've got plugged in USB wise, but I'll come to that. Since I've had this PC I've had problems (about a year now), originally I put it down to Nvidia drivers + Vista, but things just seemed to get worse, to the point where I did a reinstall of Vista last week (it was bluescreening every time I started up, running a startup repair fixed this, but problem was still there), added the latest Nvidia drivers, Setpoint, and pretty much nothing else, but I'm still having problems.

Problems manifest themselves in a variety of BSODs mixed with application crashes during games (before the reinstall games would bomb out and I'd get the Windows has restarted the graphics driver message) and lastly but not least, just system freezes (complete, no mouse movement). I've had irq not less or equal, and page fault in nonpaged area, plus various others which I lost when I reinstalled. Last two are BCCodes 101 and a.

Now, everything I saw was pointing at dodgy memory (or from some posts, interaction between memory and Nvidia cards when the memory is low quality and running max speed), so I ran memtest86 for about 10 hours and it found nothing (lots of passes, can't remember exactly how many). From what I've seen, the most likely thing that represents my problem is just bad RAM, but that should come up with plenty in memtest86 if there's something wrong.

I've also monitored my temperatures, and the CPU is running at a pretty healthy 38ish degrees, case is about 32ish. Thing is, the crashes happen when either idle or doing intense stuff, there's no immediate pattern to them, they won't necessarily happen when I'm doing intense stuff.

I've got a couple of theories. First one was that it might be the HDD, but I've scandisked it a few times and nothing at all has come up. Second is the PSU isn't powerful enough to handle the system + my USB devices (wouldn't surprise me), but I'm rubbish at comparing PSU output to devices, so advice would be appreciated. Third is the gfx is fubard, but like I said, it doesn't necessarily happen during graphically intense stuff (maybe VRAM problem?), and lastly, maybe it's the mobo, as I'm sure you can understand I'd like that last on the list.

If I have time I'm going to do some proper diagnostics at the weekend (ie swapping gfx with another one, trying different combinations of memory, etc.), but there's some stuff I just can't do (timewise and practicality-wise), such as swapping out the mobo or PSU. Any suggestions that might better focus my efforts?

0x101 BSoDs are by definition a hardware problem. It sounds like you're not overclocking, which is A Good Thing, but unfortunately that's far from the only reason why hardware might be broken.

There may be software problems too. Your \windows\minidump folder should contain a bunch of DMP files - one for each BSoD instance. If you copy a few of them out of that folder then zip 'em up and upload them here, people will jump at the chance to analyse them for you. Odd, isn't it?

It won't help with the hardware side of equation, but that approach is much, much better for software-caused crashes than attempting to guesstimate the culprit.

Like I say, it's happening with a clean install of Vista + bare essentials, so I can't see it being a software problem. I'll zip them up later and throw a couple on. From the variety of BSODs I assumed it was hw anyway, just wondering if there's any way to narrow down the culprit more.

Unless you get BSoD types which directly reference hardware (0x9C machine check exceptions, inpage errors pointing at the disk subsystem...), memory dumps rarely pinpoint the faulty hardware component(s), and that's especially true of "minidumps" which are just bare-bones summaries.

Software is reliant on hardware, and whe the hardware is broken software can malfunction in almost any conceivable way.

I'd still suggest you upload a few of those minidumps though. There may be more than one distinct cause for what ails your machine.