I'm running vista home premium on a dell laptop. My computer will only boot from the last known good configuration. If I go to the logon screen and try to type a password it is rejected and the computer reboots, if I pause before entering the password (10 sec). the computer reboots. I can't enter safe mode and system restore shows no active restore points. I have run malibytes malware, and it did find and remove some infections but didn't help the boot problem. I am running treadmicro anitvirus and stopzilla spyware programs neither report any problems. This problem seemed to occur after trying to download vista sp1, I started the download and just let the computer run,when I checked it later it said download had failed and since then I've had the problem. I checked under installed upgrades and sp1 is not listed.

A couple of things to try:
1) Startup repair: http://www.bleepingcomputer.com/tutorials/tutorial148.html
2) A hard drive diagnostic from the manufacturer of your hard drive.