Easy enough. Just click on the Utilities menu at the top, start up Disk Utilities, and create a partition on the drive. You may have to restart the installer, but after that it should let you proceed.

And minor point, but it might save you some time down the road if looking for help. The CPU is one component of the overall computer, it's not the tower itself. If you say something about the CPU having been purged, it may throw someone off, and then you both waste a few minutes trying to figure out that you mean the computer hard drive.