Great news! I finally found a solution to my drive issue! It took me several hours of nonstop marathon troubleshooting, but perseverance really pays off! I could go on for pages describing everything I tried, but for the sake of simplicity, I'm going to just explain the winning moves. I downloaded EasyBCD, and set up the bootloader so that both Windows 7 and XP could boot in harmony. Unfortunately, I was still getting errors when I tried to boot into XP. So what I basically did was boot up from my XP install disc, launch the Recovery Console, and executed FIXBOOT. This fixed the XP partition, but now 7 wouldn't boot, and it was basically ignored by the bootloader.
So then I booted from my 7 repair disc (it's a damn good thing I have all these discs lying around!) and ran Startup Repair. After it fixed the boot sector, 7 booted fine, but now the reverse was true: XP was ignored. This is what I meant when I titled my post a flip flop! However, with the boot sectors now repaired with XP, all I had to do was add XP as an entry to my EasyBCD bootloader, and now I can select which drive I'd like to boot at startup, without any problems at all!
This is a unique situation, because previously - when everything was working - I would boot into either 7 or XP depending on which I had set to boot first in the BIOS. There was no bootloader to prompt me on a choice: both installations worked, I just had to specify which one I wanted to boot from the BIOS boot menu. In some ways, this is more convenient, because now I don't have to use the BIOS menu. On the other hand, if I wanted to boot into, say, 7 as the default, I either have to wait 30 seconds for the default drive to automatically boot, or I have to remember to press it after I start up the machine.
In any event, I'm just grateful I got both systems booting normally again. I've certainly learned my lesson: no more playing with the System Reserved drive for me!