How to make primary HDD with OS boot

I have two HDD's. One is an IDE HDD with Windows XP on it. The other is a SATA HDD with Windows 7 just recently installed on it. In the BIOS, it tells me that the IDE HDD is the primary HDD and it needs to be first in the Boot Device Priority - if I change this, I have an error on startup telling me that I need to change the Boot Device Priority, and this error only goes away if I make the IDE HDD first again.

What I want to do is to make my SATA HDD with Windows 7 be the Primary HDD, and for it to boot first. I want to disconnect my IDE HDD because I don't want to use Windows XP anymore, and want to purely use Windows 7 on my SATA HDD.

I tried (and failed) to use Powerquest Partition Magic to forcibly change the drive letter name on the IDE HDD from C: to something else, and to change the SATA HDD to C: but this change only works when I choose Windows XP on startup and then it subsequently fails to boot Windows XP - but choosing Windows 7 on startup works fine. This is because under Windows 7, the C: stays the IDE HDD unlike in Windows XP. I have another thread on this problem here: http://www.tomshardware.com/forum/244030-45-changed-let...

I would really like to get to the bottom of this, and I have been working on this for hours. I have a good amount of tech experience, and I have googled this extensively but I can't find anything that addresses this particular problem and I seem to be at my wit's end. Any help is very appreciated!