In BIOS, make sure the SATA controllers are set to either "Legacy" or "Normal" (the wording varies by board). If they are set to "AHCI" or "RAID" the system will not boot. Usually there is an error but a flashing cursor wouldn't surprise me either. By setting the controller to the Legacy/Normal mode, you are telling it to emulate how the IDE controllers work.
If that doesn't work, you might need to boot off your windows 7 disk and run the repair tools on the install by clicking the "repair" option on the first screen. Usually after disk swapping it does the boot-sector fixing automatically.