After you have installed and ran Windows 7 for a few days, you may want to run Disk Cleanup to reclaim some space taken up by the install program.
On my ‘clean’ install of Windows 7 RC1, I had 847 MB of space to claim back, not including 15.2 GB that the Windows.old folder occupies. If you are 100% sure you do not need these files, check the box and Disk Cleanup will delete them. You may want to copy the Windows.old folder to another partition or an external drive first, just to be on the safe side.
Then, after Disk Cleanup has finished its job, run Disk Defragmenter to optimize your hard drive.