dell inspiron e1505 xp home 2Gram, 80g hard drive.

when it starts, never finishes the bios and won't go into windows start page. ran diagnistics, everything passed. tried to run repair disk after loading windows files, when it goes to restart the windows....it won't. all it is doing is start the bios and ga back and start bios again aand again...chked hard drive, mother board and everything seems ok and passing according to dell diagnostic cd. It was on for a week with web cam runing but when i decided to shut it down it won't start now. I tested bios battery too and it is ok, 2.98 v. Any help for this problem will be appreciate it. last year I redid the windows and it has been fine but my C: drive letter is D: now and my backup is C: