Hello guys, i've been reading this topic, and i wanted to share how i fixed this problem by your inspirations...
I had a Vista 64bit OS, then i added a winXP just like everyone else did it.
But then i installed the vistabootpro 3.3 freeware version on VISTA, not on XP.
Then, i started the program and guess what, it only shows one OS, it shows VISTA, doesn't show XP.
Then inside the program from the top Diagnostics menu, i clicked Run Diagnostics.
And after this, i clikced Manage OS Entries section, and now i had 2 OS appearing. One VISTA, and the other one "Former Windows-Version".
After that, i went to Systen Bootloader section, and from the first group of selections i chose Windows Vista Bootloader selection. And from the second, Specific Drive and set c: as the drive. I left the "Force Bootloader Install (Optional)" option unchecked. Then i hit install bootloader.
And then, when i restart, i had the OS selection screen, One VISTA, and the other "Former Windows-Version"
This method worked for me. Hope it helps.
Greetings to you all from Istanbul.