Vista - Vista Starup Problem

Hi, new here, and didn't know where to put this, but here goes (sorry long):

I dl'd Mozilla Firefox a day ago, and everything dl'd correctly and completely. But, when I shut down the cpu, it wouldn't start up at all.

I did every diagnostic i could let the cpu do, ran a system restore (went all the way back to 10/25/09 restore point). then, i got it to run in safe mode. played with start-up options, and got the desktop to come up in normal mode, but then got error msg 0x800106ba (windows defender won't load), this meant that none of my security stuff loaded either. i ran a chkdisc thing too, and it stopped when looking through file data (stopped at 16%).

thus, i am running the cpu in safe mode with networking, and the cpu is running fine.

i know this means that there is a file issue. but i am not sure how to find it.

please help.

thanks