I have WinXP SP3 32bit installed on a raid drive. I have a spare SATA drive I added to my PC to install Windows 7 64bit on for dual boot.
Becase my WinXP is 32bit, I can't initiate the Windows 7 install from within XP. I reboot with the DVD in the drive and the install kicks off. I select "Custom Install" and everything appears to work fine, but my boot loader isn't getting updated for dual boot purposes. As long as I keep rebooting, Windows 7 loads. If I turn the PC off, I now default back to WinXP.
I can go into CMOS and change the Hard Drive priority in the boot section from one drive to another to force that OS to boot, but that's obviously a pain.