...then you have few options other than formatting and reinstalling everything back to its factory condition.. Unfortunately, you've not told us anything about the computer.. Make and model, laptop or desktop, etc. And although you mentioned a "repair" option in F8, did you try pressing F8 immediately after startup till a selection screen loads, then use the up/down arrow keys to select "Last Known Good Configuration", then press "Enter".. It should perform a System Restore back to a time when things were good, unless system files have been removed..
If system files have been damaged, or removed, and you have no recovery discs, then many brand name computers have a hidden Recovery Partition which can be used to format and reinstall everything back to factory state.. But since you haven't indicated specifics about your computer, this is about all I can do till more details are given.
Hope this helps and let us know more.
I have been constantly getting a blue screen error message about an executive worker in memory (?) and my acer laptop would shutdown and then reboot automatically. but then i got a spam email that caused a blue screen error stating that window wasn't working properly. then i couldn't reboot at all unless it was in safe mode. i figured it was a virus so i downloaded avast and did a scan which came up with 27 error messages. i got rid of the viruses, but then when i went to reboot windows normally, it just stops at a black screen with the pointer. i dont have any of the original CDs that came with the laptop, and when i did F8 for the repair option, at one point i was asked for username and password and it wouldn't accept the ones that i use to log into the laptop on a regular basis so that didnt help. is there anything i can do other than having to do a clean install of windows????