XPS 420 upgraded cpu vista will not boot past post

another nightmare, I have upgraded my cpu on the xps 420 and it booted up ok after the first time I upgraded cpu, I also upgraded the graphics card and then added the latest drivers from nvidia website, I then though that I had not applied enough thermal paste so cleaned the cpu and took it off and the reseated the cpu back, then the nightmare of windows vista just goes to the post screen with the green bar at the bottom and the bar only moves slightly and the screen goes blank and I see the blue screen but cannot amke out the information as it restarts immedietly, it then gives me an option of recovery or normal mode, if I click on normal mode it does the same, if I click on the repair it loads up and gives a few options like repair which it does and the states that windows could not fine any probs, I have also used the restore  but to no avail, I have also tries this repair from the dell vista cd, and when I goto repair it does the same stating windows cannot repair as no problems have been found, I can see everythig on my hard drive inc all programs etc when I click on the backup option on this and also x drive (which aprt from the dvd and the c drive is also there) I have chkdsk/r but nothing it still does the same thing, I have then changed the cpu to the same one and also the graphics card to the 2400 pro but still exactly the same, I donot want to reformat the drive as I have vast amount of updates I had loaded from microsoft and also a lot of work, is there any help on this, I have also taken the drive out and added a new drive and installed vista no probs, it is working, please could any one help on how to fix the first drive without reinstalling windows vista. I have also done all the diagnostics etc for the whole system tand everything is in working order inc memory which I have downgarded to 2 gig

Dell XPS 420

QX6800

8800 ultra superclocked

corsair HX520 PSU

crucial 4 GB memory

500GB hard drive

BIOS A06

windows vista ultimate