Windows 7 (64 bit) bootmanager fails Status: 0xc0000225

When trying to boot I get the "Starting Windows" but the PC hangs then - no HDD activity, power off is the only course.
In Safe Mode the PC hangs after loading CLASSPNP.DYD

Attempted repair fails with Status: 0xc0000225 "boot selection failed because a required device is inaccessible"

I tried booting off the installation DVD but after loading Windows files it hangs at the "Starting Windows" stage as above.

I am able to boot off a Memtest CD (that shows no errors), also off a general diagnostic (Ultra-X QuickTech Pro) CD that shows no errors in any component.
However, trying to boot off a Knoppix CD hangs leaving a blank screen. I have restored the HDD from a backup image. The PC comprises mobo, HDD, optical, PSU only and all cables look good and tight.

Having read other posts with the same problem I looked through the BIOS but all seems good there; I did set to default values but problem persists.

Thanks for the reply Lordbob.
Alas, I wiped the disk and copied a backup over it so no previous dumps exist, and I don't think Windows is getting far enough to create one now.
All cables seem fine, and the diagnostics all worked when booting from a CD.
It's a head scratcher of a situation.