This really looks like something is trying to start up when you reboot and not being able to.
1. click start, run and type msconfig
2. in the window that opens slect the start up tab. here you will see all the programs which start up when you boot windows.
3. remove the tick from boxes you know that you will not need when you start the PC. If you are not sure what they do leave them or google it to find out.
4. click apply and close the window, windows will prompt for a reboot to make the changes.
When windows reboots try and see if the problem persists. if it does not then you will know that there was a problem loading software at start up. the trick now is to find out what was causing the problem. I would normally reinstall any software which I prevented from starting up, or if not required remove it.