Success!!! .... Thought i'd share my dealings with what I did to get 7 and XP booting together on 2 different disks:
1) Unplugged my Windows7 disk
2) Put in spare 80gb disk, changed HDD priority in BIOS, and installed Windows XP x64 on it
3) Put my Windows 7 disk back in and changed HDD priority in BIOS back to my Windows 7 disk
4) Booted up to Windows 7, loaded EasyBC, added an entry for Windows XP
5) Went in to Change Settings > Entry Based Settings > and change the disk from C: to F: for Windows XP entry
6) Rebooted and walla!
Got a nice little menu that appears after BIOS screen asking for Windows 7 or Windows XP x64 and both work like a charm! Also changed the boot time wait from 30 secs to 5 secs.
I also installed iReboot so I can now reboot to 7 or XP from the tray icon!
Oh, and for all those that were wondering why I did this, it's because despite many work-around attempts, I cannot get COD4 or PunkBuster to be precise to work under Windows 7 at the moment and I play a lot of clan matches ... now when I want to play I just reboot in to XP and off I go ... it's just a fresh install of XP SP2 x64 with COD4 / Ventrilo installed with some freebie AV suite ... fresh install of XP runs rather quick on an Quad Core 3Ghz with 4GB ram lol ...