Rebooting all the time

The computer starts normally with one beep, until it returns a black and white screen saying that software or hardware changes have been made in the computer since I last used the computer. Then it gives me 5 options:
1. start in safe mode
2. start in safe mode with networking
3. start in DOS
4. start with last known good configuration
5. start windows normally

The first time, I chose option 4 and I arrived at the Windows logo but then it beeps again and restarts the computer. After this it comes backt to the smae black and white screen as before, this time I choose option 1 and the same thing happens again.
What should I do? Is it a trojan? I do not want to have to format my drive. Please help.