I already have vista 64 installed and every time i try to istall xp, the system only recognizes xp and not OSS. How do I GET the computer to recognize both?Oh, and they are on seperate hdshttp://apcmag.com/how_to_dual_boot_vista_and_xp_with_vista_installed_first__the_stepbystep_guide.htmMostly it's unavailable to install Windows XP after Windows 7 because it's a HUGE propability that errors may occur, such as

" Disk boot fail"
 Press CTRL+ALT+DEL to restart

your Boot Manager will not be able to boot Windows XP, etc. I spend all night in order to figure that out and the results was failure all the time. But I found an even easier way to do it but it will cost you a HDD format.
Follow these few steps and I hope they will help you.

1) Format your C: HDD
2) Split the C: HDD into 2 partitions.
3) On the first partition install Windows XP.
4) On the second partition install Windows 7, SINCE the Boot Manager is much newer than that of Windows XP.
5) When you manage to do these, whenever you will boot your PC, the Boot Manager will show you 2 options:

Older version of windows (which is windows xp)
Windows 7

so now you can choose which windows you will operate.
It worked for me so I think it will also work for you. Ignore the above post and follow the instructions in the link I provided above.Couldn't agree more... Quote from: pcjoseph1974 on July 01, 2010, 05:45:01 AM

Oh, and they are on seperate hds

If they're on two drives then you can just change your boot order in BIOS depending on which one you want to boot.