It seems you have removed the drive that had the Windows boot files, so the system can no longer boot.
With the old drive disconnected, boot from the Windows 7 DVD and run startup repair several times.
Or fix it manually using the command-line. Some examples:
Shift + F10 bcdboot c:\windows bootsect /nt60 SYS /force
Also, refer to this link: How to use the Bootrec.exe tool in the Windows Recovery Environment to troubleshoot and repair startup issues in Windows 7
You can't dual boot when one of the drives is disconnected. If you are planning on disconnecting drives then you need to make sure that both disks have the necessary boot information. Also you probably need to go into your BIOS and change the order in which the computer tries to access it.