windows vista keeps rebooting

Hi,
I just realized that when you conect a hard drive to another system its going to have another drive letter other than the main C: drive letter that belongs to the system.

My main problem is windows vista keeps rebooting right after it gets to main log-in screen and it keeps doing that in a loop. How can I fix this without losing the data?

Boot to safe mode and look at the system event logs.  

I note you use the word "rebooting" - do you mean shutting down and restarting or bluescreening?  The bluescreen will tell you what driver caused the issue.  Boot to safe mode and simply rename that file, then try again. If it's a virus/trojan or similar, get autoruns and RootKitRevealer onto the system, let RKR scan it, and use autoruns to prevent nasties... or just the piece of crap that's causing the crash from starting.  It'll tell you where they're launched from too.

yes shutting down and restarts. I tried safe mode and it makes it to the log in screen and when I type in the password and hit enter it shuts down and restarts.

run a startup repair :  http://www.bleepingcomputer.com/tutorials/tutorial148.html      repair Vista

also, post exactly which one :  "and ran a diagnostic and it passed"

I tried startup repair and it did not work. I also tried to do system restore and that did not work. The laptop turns on fine however when it gets to the log in screen of vista just before the password box where you have to type your password it shuts down and restarts. It keeps doing this in a loop everytime.

>>  it did not work  << yeah, but what happened?  any message or error?

also, what Diag? :  also, post exactly which one :  "and ran a diagnostic and it passed"

the start up repair did not work. I got a message saying it did not find a problem. then I tried system restore and I got message stating that it could not find a restore point. the diagnostic I ran was the dell diagnostics. the reason I wanted to repair windows if there was a way to do it was because there are some software installed that I do not have the cd for it and also avoiding to have to re-install everything..

Hi,
I am so sorry I forgot to respond back. I reformated the hard drive and reinstalled the operating system.... thanks

tx for feedback