If you want any more in depth help you'll need to fill in your System Specs in bottom left corner of post.
You can first enter BIOS setup by tapping the key given on first screen. Look for the HD or storage section where you might be able to disable one HD without having to unplug it. Look also for the SATA controller setting to change it to AHCI.
If you can't disable a HD, then power down to open the case, pull the data cable off of one of the HD's to install to the other one.
Now boot the Windows 7 DvD, at first screen press Shift + F10 to open a Command prompt, Type:
SEL DISK 0 (replace 0 with the Windows 7 target HD number)
Now close the Command Box, click Install Now and proceed with Clean Install Windows 7
using Drive Options to repartition, format and install, or just click Next at drive selection screen to have installer do it for you.