Vista and XP each on seperate HD.

I have Vista installed on one HD and XP on another. I can boot into each OS through BIOS by setting which HD boots first. Is there a way I could dual boot without partitioning a single HD?

If you installed them in the proper order, while keeping both drives in, you'd get a menu choice for either OS. Or, even better, you would use a VM application, and never have to mess with multi-booting again.

I wouldn't use Microsoft's very limited and convoluted method of multi-booting, which integrates boot files and creates a dependency between OSes. Problems can arise between OSes when they are not properly hidden from each other. In the event of a boot problem, both OSes will be affected.

With a third-party boot manager, you can have each OS on its own drive be self-sufficient and operate fully independently if one drive is removed or fails.

Go here for some excellent information on the basics of multibooting:

http://www.goodells.net/multiboot/

Without partitioning any of the drives, I guess one viable option would be to install a boot manager on a USB flash drive and boot from there, leaving each OS drive fully intact.

You might also want to consider transferring the XP installation to a virtual machine. If you need 3D acceleration, VMware Workstation 6.5 supports DX9 graphics, but with a significant penalty to native performance.