Hi I have a similar problem, but it occurred in a differnet way.
I had XP installed on one disk, with another disk for data.
I then wiped the other disk, disconnected the xp disk and installed 7 on the now empty disk.
If I connect back up the XP disk, it always boots into XP, unless I go into the bootload screen for my MOBO, and select the 7 disk to boot from.
How can I get the bootload screen options in this case please?