It is a bad idea to install the Win8 CP over your Win7 system. I suggest you define an extra partition for Win8 and install into that partition. That partition can be a logical partition. Then you get a double boot for both systems.
Be careful with the definition of a new partition though (even a logical partition). Check first in Disk Management whether you have only 3 (or less) primary partitions on that disk. If you have already 4 primary partitions, you have to first convert one of the primaries to logical, else you get a BIG problem with dynamic partitions.