My problem is I have not encountered your situation, so I have not been able to test how to get out. I might suggest some things, but it could cause other problems for your system.
So, this is up to you. If you do not want to take a chance, do not do this.
Boot to the Install DVD, go to the second page and choose repair. After it does its thing, go to a command prompt and type the following and hit enter after.
Bootrec /fixmbr
Bootrec /fixboot
Close the window and try a reboot. All I can think to do is to try to break the rebooting loop. I do not know where the reboot requirement is kept, so I do not know how to get rid of it.
If you can get anywhere with a command prompt you might also try typing explorer.exe, or taskmgr.exe, or msconfig.exe to see if you can get something running you can use to change the boot options.
I will also pass this along just in case it might help. I have not tried and it does seem to be for Vista, but Win 7 and Vista are almost the same.
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