Here is how I defragment my C drive:
1. Make disc image backup to external hard-drive.
2. Checkdisk to recover those bad sectors.
3. Restart.
3. SuperCleaner (because it overwrites).
4. CCleaner.
5. RegSeeker: "Clean the Registry".
6. Restart.
7. Eraser "unused space wipe".
8. Restart in safe mode and defragment.
9. Restart.
10. Make disc image backup to external hard-drive.
The first time I ran it on my computer (a 2 month old install) it was around 650MB.
The first time I ran it on my GF's computer (a 3 year old install) it was around 550MB.
That is like saying Ad-Aware SE Personal edition prevents spyware. It helps to remove spyware but I do not feel that it prevents anything. If you want to stop programs from being added to "your HKLM Run part of your registry," I would recommend installing StartupMonitor.