Boot up the computer with the win 7 disk in the cd drive. You will/should be prompted to hit "any key" to boot with the CD drive. You have to watch it, because it'll only last about 10 seconds and then it'll just boot up off the HD and you have to start over if you don't catch it in time.
Once you boot up on the disk, it'll load it's files and such and start the install wizard.
At some point, it'll prompt you for the partition you want to install to. You have to DELETE the existing partition first, so look for that option first.
Once it's deleted, you will need to install to that UNFORMATED partition and you'll have to tell it the size you want it to be... Just use the max size it says that's available.
At that point, it should format the partition AND install a fresh copy of Win 7 on that drive.
Make sure you also have your computers DEVICE DRIVER disk handy, as you'll need that to load the video, audio, network, wireless, etc. specific to your computer.
Once you get your basics loaded, you can get on the interent for the windows updates (install the Win 7 service pack 2 first) and then load up on the rest of the updates.
Another thing I like to do is do a DEFRAG on the hard drvie after you get windows initially installed..
Good luck!
Mitch