Sorry, bad phrasing. If you reinstall without formatting, you'll have to delete a bunch of stuff, run defrag, etc... but if you do a clean install, which includes a format, you don't need to defrag because the old file table is wiped out, and there's no extra deleting to do, since the format handles that. The downside is, if you forgot to back anything up, it's pretty well gone.

If you do format it, you should consider using the partitioning utility to make a "system" disk and a "data" disk, so that if you reinstall again it won't touch your data files.