Open "My Computer", right click on the C: drive, go to properties. Choose the second tab over ("Tools") and try "defragment now"
Barring that, try checking under Windows add/remove programs, under the "add/remove windows components" button and make sure defrag is listed as one of the installed components (I can't imagine it wouldn't be, though. As far as I know it's part of the operating system).
If that doesn't work try booting into safe mode (hit F8 repeatedy during boot up, after the POST screen but before windows starts loading) and run it from there.