The disk boot error is simply forgetting to reset the hard drive as the first in the boot order. Too often people go into the bios directly rather then simply bringing up the boot device menu to select the optical drive while leaving the hard drive set as the default.
The F1, F8, F12, or other F key is assigned to boot floppy, cd/dvd, or hard drive if the bios on your board sees a key assigned for that. It makes life a little easier since that's the one time session needed for seeing all setup files copied over.
Once you install XP or Vista onto the second partition the mbr information for 7 needs to be repaired using the startup repair tool. For seeing a 7/Vista dual boot refer to
Dual Boot Installation with Windows 7 and Vista
With XP installed after 7 it works the same as you would see with XP following Vista since the boot loaders for Vista and 7 are the same type. You need to unhide the XP boot files to copy them to the root of the 7 primary. An edit of the boot.ini copy may be needed at times while it should show rdisk(0)partition(2).