Make a System Repair Disc on another W7 PC, it needs to be the same bit version as the one you are repairing, 32 or 64bit Windows 7.
Once you have the System Repair Disc,
Insert the Windows 7 installation or System Repair DVD and boot from your DVD drive. You may have to change the boot order through system BIOS to boot from your DVD.
Choose your default "Language," "Time" and "Keyboard Input" on the first window and click "Next."
Click on the "Repair Your Computer" option to gain access to the System Recovery window. Now choose "Command Prompt" to run the Bootsect.exe utility. Bootsect is located inside the boot folder so change your directory to boot. Now run
bootsect /nt60 C:\ if you had Windows 7 initially installed in the C partition. Alternatively, you can run
bootsect /nt60 SYS or
bootsect /nt60 ALL to repair the system partition or all partitions. Eject the DVD and restart your computer. Your computer should now boot Windows 7 again.
As stated in the comments below, one has also to use the
/mbr option in order to fix MBR, which is necessary to remove GRUB from MBR.