Right here goes.
You put in the CD boot from it.
You'll get to the point where it says we found a pervoius version of windows would you like to repair it press enter (i think)
Would you like to install fresh copy press esc(i think)
it takes you to screen with with all your partitioned hard drives.
the installation your using now would be on C;\ drive.
So you pick different hard drive E:\ or what ever providing you have 2 hard drives in your computer. if not you must have current hard drive split in to(can only be done during the install of windows for hard drive one.
windows can not install on same hard drive twice ( or technically same partition)
so even when it says you have previous version just hit esc. then select a different hard drive/Partition for installation.
when your PC boots it will give you the option of what windows to choose 1st one or the 2nd one
Technically you probably didnt daul boot you probably reinstalled windows and it pretended like you had to versions. (that is installation bug were it doesnt remove first entry in the boot.ini lolM$ is for u)