Windows Vista STOP: c000021a Fatal System Error

Hi All, I wonder if anyone can help please? I've broken my wife's PC, which is obviously bad news! Here's the problem:

It's an Acer 5101AWLMi laptop running Vista Home Premium, and it suddenly started handling all types of media very slowly, to the point where just viewing stuff on youtube was painfully slow. I removed various bits of software such as free toolbars, media players and anything which was not critical for the system.
I then went to the Acer website and downloaded the latest ATI graphics drivers for the X1100 chipset in the laptop. It made no difference, so I removed the update.
I then rolled back the ATI driver to the previous, previous version (which said it was X200 I think) and went into the garden. When I came back 10 minutes later, the machine had re-booted automatically and was saying that there was a problem and would I like to boot Windows in safe mode. I tried to reboot using all options, but get the same STOP error:

STOP: c000021a {Fatal System Error}
The initial session process or system process terminated unexpectedly with a status of 0x00000000 (0xc0000001 0x0010037c).
The system has been shut down.

My expertise is limited. I have no back-up disk, and have yet to find any Vista OS discs. Does anyone have any ideas please? Actually, I've just selected the Repair option again after unplugging all usb devices and the power supply, and it's started doing something. I'll post how it goes just in case it helps anyone else.

Thanks Therixon.

 

That didn't work. It looks like it's a corrupt file, and Windows can't fix it.

 

Would you believe it, after hours of blue screens, it's finally managed to boot properly. It's also put the ATI driver back to correct (albeit old) version. However, the initial problem still persists, with slowdowns in everything web-based, or media based. Even had to do the online supermarket order on a different machine.

 

Right, I've just completed the Vista performance index, and it says that the Graphics capability has gone down from 3.1 to 2.9. Everything else is the same, so I think it's the driver, however Vista also thinks that new hardware was added between the last test and this one.