Windows 7 - computer restarts every 1 hour and say "Boot device not found"

Since yesterday, my computer hangs and restarts automatically approx every 1 hour approximately. Also occasionally, the system does not boot and fails with the error "boot device not found".

I don't remember installing anything new. I tried looking at the Task Manager just before it freezes and found nothing suspicious. EventLog has no information. It just says "Windows unexpectedly rebooted" after reboot, but there is nothing when it freezes. I cleaned up the startup tools using "msconfig".

Is this some kind of a virus (boot sector virus) as sometimes it is not able to boot. My hard drive is an SSD and I did chkdsk and found nothing.

Is there anything that I could do without formatting and reinstalling?

Check the STOP code on the blue screen as suggested by DragonLord. If the error indicates a hard disk problem, check the firmware version of your SSD and install the latest version. I had similar problems with an OCZ SSD, which were resolved by updating the firmware.

You can try pressing F8 during boot and rebooting from the last known good boot in the menu selection.

You might also try using your Win 7 CD, booting into recovery mode and restoring from one of the save points.