Vista - Random Memory Errors at Logoff (and ONLY Logoff)

I hope I've posted this in the correct place and I'm sorry if I didn't but I did search the forums to see if my questions had been answered and I couldn't find anything.

okkay, Here's the problem. Every now and again when I log off or log off and shutdown
I get a memory error like: 'explorer.exe referenced memory at address (random number I can't remember but their different every time.). The memory could not be 'read'.'
I do not get any other error messages at any other time or from any other program.

Then Vista finally logs off after that error and that's that. It's happy again. I have already run Memtest 3 times, scanned for viruses & malware, scanned for invalid reg files, physically cleaned up laptop from dust so my over zealous CPU doesn't overheat and made sure the memory sticks didn't have any physical gunk in them, Reformated and reinstalled Vista, and still this error occurs but not every time I logoff. just sometimes.

Do any of you guys have any idea at what else could possibly create memory errors like this. Even remote and crazy theories are welcomed because I just don't know.

Oh, also, I do have QTTab installed for tabbed browsing of my explorer. Maybe that's it. But I swear I still got the errors before I first installed it. So I don't have clue. But perhaps another program is doing it. Maybe those reg cleaners are doing it. But I don't see how. So, Help! What am I missing?

Empty your temp files. Run chdksk (error checking) and defrag your hard drive.

thanks for the suggestion and response.
already did all that too and chkdsk did find some wayward free clusters but that was before I actually got my first error message after I reinstalled my OS. And you have no idea the effort I put into defragging my drive so I've done that too.
But it couldn't hurt to do another chkdsk just to be sure, so will do.

Get O&O Defrag 10 or 11. It is what I use. It's much faster than defrag. If you defrag more often, it takes less time. I do it a few times a week. My workstation runs much faster.

Okay, I'll try that O&O Program. I've used almost every other defragger out there but I haven't given that one a try. And I actually use Avanquest JETDefrag about 3 times a week and Auslogics Defrag or Smart Defrag or Defraggler about once or twice a day. I do it that often because I do manipulate, create and delete lots of files on a almost never ending basis. Like GB's worth of music, video and picture files.

You know, it just occured to me that I started getting these errors right around the time I switched some unnessaacery services to manual or disable. I think i'll fix that but I'm not convinced that's it.

Okay, off to get O&O Defrag...