I had a different scenario, I had Win7 RC installed to a 2TB RAID5 volume, I do not have space to backup all that is on this volume (I only backup the important stuff)
So to go around the option of deleting the partition I used my own approach of simply deleteing the files, "simply" is putting it lightly though
It goes without saying that these commands must be run from the OS you do not intend to delete, in my case that is in Windows XP x64 deleting Windows 7 RC
Windows7 bootloader is on my C: drive, Windows7 is on my D: drive, keep this in mind for the commands below and modify for your own system!
From the Vista or Win7 CD find the program bootsect.exe, run it with this command; bootsect.exe /nt52 c:
This replaces your bootloader with that of Windows XP
Now to delete the bootfiles do;
takeown /r /f c:\boot
cacls c:\boot /T /G Administrator:F
After this you should be able to delete the folder "boot" in explorer, now do the same for the other Windows folders, like;
D:\Windows
D:\Users
D:\Program Files
D:\Program Files (x86)
On the next reboot you should hopefully get the Windows XP bootloader directly, and not even the choice of starting Windows7
Of course doing the above with no backups means you must really know what you are doing, one wrong move and you could loose all your data!