Another option is to get into the case and look for the CMOS battery which keeps the startup options for your PC. You can take the battery out (just clips in) and replace after 30 seconds or so and try booting again.
Sometimes this causes problems as well.
Also, do you get any “POST” beeps when you start up? These are the beeps the motherboard gives out that can tell you where problem lies - ie 3 short beeps and then 1 long, 3 long beeps, etc.
POST Codes can identify which part of the hardware is not cooperating, or even if it appears to be a hardware based issue.
One beep and then to the windows xp screen
the scan screen
then to the inconvenience screen
then looping
What is a cmos battery?