I have multiple Win7 OS's installed on 1 hard drive.
What I do is defragment the Hdd.
Go to Computer management (rt. clk. computer/Manage/ Disk Management)
Rt. clk. on the Hdd & choose shrink volume.
Go through the wizard choosing the amount to shrink the Hdd., use the default drive letter (ex: "F").
You'll have to let the PC restart.
Back @ the desktop go to Computer management again & rt. clk. on the new partition and make it active.
Now restart the PC w/ the DVD in the drive & boot from that drive & install Win7 on the new partition you created.