You need to edit your boot.ini file try this. It's based on XP but should be similar if not the same in Vista.
Right click on My Computer, and select Properties. Now click on the Advanced tab. In a section labeled Startup and Recovery click on the button labeled Settings.
Click on Edit to edit the startup options file (aka boot.ini) manually. That should open Notepad with something similar to the following: (you may want to make a backup before altering anything)
[boot loader]
timeout=5
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptOut
The above has only 1 option on boot: Windows XP. I guess yours has XP, Vista and Win7 .You can simply delete the Win7 line and save the file replacing the original.
That should have solved the problem on next boot.