The computer should, if it didn't come with any disks, have a recovery partition set up.
So, First try to get to the Advanced Options Menu:
http://www.sevenforums.com/tutorials/666-advanced-boot-options.html
If you can't get to anything there, you will need a boot disk to launch the Windows 7 recovery options:
> If you have another computer with Windows 7 then you can create a System Recovery disk:
http://www.sevenforums.com/tutorials/2083-system-repair-disc-create.html
> if you don't have access to a Windows 7 computer, but can create a disk on another, you can download a copy of Windows 7 and go through the steps to repair from that disk:
http://www.sevenforums.com/tutorials/219487-clean-reinstall-factory-oem-windows-7-a.html
From those, you can run the Startup Repair.
Or use the C Prompt to reset the Original Partition to active:
http://www.sevenforums.com/tutorials/71432-partition-mark-active.html
Hope this helps