Repetitive blue screen problems

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I could do with some assistance on some blue screen problems I have. This is on a 2yr old Vista 64 machine that's been running perfectly fine up until now.

The history is as follows:
Regular BSODs were occurring - 90% of these were "IRQL not less or equal"
The screen would flicker and then it'd BSOD
The PC could restart normally and in safe mode but would BSOD during use (sometimes soon after the restart, other times after hours had passed)
No new software had been added, no drivers updated for weeks
Temperature was fine
Chkdisk /r found nothing wrong with the HDD
SeaTools found nothing wrong with the HDD
Memory checks found nothing wrong
PSU checked out fine
Device manager said there were some issues with the CD/DVD drivers but it worked ok and Windows was stating it had the latest drivers
Things got worse - now it won't boot at all - "21a session manager failed to create protected prefixes system terminated unexpectedly with a status of 0xc000003a" - this always occurs now so I can't get into Windows
The NeoSmart Vista recovery CD boots ok but throws an unexpected I/O error later. The CD checks out ok on other machines and I've tried using an external, USB DVD drive too but with the same result.
All combinations of memory tried - one stick, two sticks, different slots etc
All peripherals unplugged
Disconnected HDD and tried to boot from CD - still get the I/O error
Disconnected DVD - still get the 21a error

I've used a number of bootable tools and they've found no issues (and would also suggest the DVD is fine) but I can't get the Vista recovery disk to run all the way through - I get that I/O error.

So, I can't get into Windows to do more extended diagnostics and I can't get the Vista recovery console up in order to examine things from outside. I've got the Windows 7 recovery disk now and will try that shortly. I also have the Windows 7 evaluation installer and will give that a go.

Does anyone have any ideas as to what could be going on here? Or on additional plans of action?
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I know this post is 6 years old, but just went through the same thing. At first, used a commercial online company, and for 29 bucks and an hours of playing with my computer from India, they did nothing but offer to fix it for 500. A bit more reading, and pretty sure I got lucky and solved it myself. My comp would attempt to load Windows and would usually stop at the point the whole desktop populated, and then went to blue screen with the IRQL excuse. I could load up in safe mode though. I did the usual recommended stuff, removed programs, updated drivers, etc., that seems to be a waste of time though. But, running 'msconfig' is how I solved it. I use Microsoft Security Essentials, and it had been updated on Aug. 3rd, and the problem arrived on Aug. 4th with a binary file I foolishly ran, thinking it was something else. I ran msconfig, opened the 'services' tab, and just started to pick and choose various things to not have run, and restart. Gave that up and just unchecked the first ten items, and restarted, same problem. Went back and unchecked the next ten, same result. On the third try, unchecking the next 10, I thought I had really screwed something up, nothing was happening, not even an attempt to load windows. After my oh no moment, a little screen popped open and it was MSE saying they found something wrong. After a couple minutes, windows started perfectly without issue. I installed an updated version of Malwarebytes, being one of the programs I had removed in my earlier trouble shooting, and ran it, it located and removed over 5000 bugs of various kinds. Not sure how the binary file got past MSE, thought that should have caught it, but it didn't. Apparently, in my unchecking those start up programs, the bug was in one of them, and MSE was able to locate and remove it since it did not start and prevent MSE from running initially. I know this has to be pure luck, maybe I'll go get a lottery ticket now.
Glad you solved your problem.
Beware of fake computer support companies that pop-up when searching for help.
Locking this thread due to its age.
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