I have Vista on C, and XP on logical drive D (same physical drive). I had originally had Windows 98 on C drive, so as you can imagine I had a little fun getting it to work. But VistabootPro saved my bacon.
I needed to boot XP in Safe Mode tonight. When I have Vista selected in the boot menu, I have the option to press F8, but if I have XP highlighted, there is no such option. I tried hitting F8 quickly when I chose XP, but it still goes into normal mode.
The only way I ever managed to get it in Safe Mode was unorthodox. I let it start booting in Normal mode and then shut the computer off. Then when I turned it back on and chose XP, it automatically brought up the start menu. Is there an "orthodox" way of booting XP in Safe Mode?

Try pressing F8 before the OS selection menu comes up.

In any case , it should provide you with the XP boot menu selection after you have selected XP , pressed Enter and then hit F8 ( very quickly ).