It's easy but need some time.
1. Lavasoft - Ad-aware SE personal
2. Spybot Search & Destroy
4. Antivirus software(any)
Just follow these steps:
1. Perform the disk cleanup.
2. Install all the programs above
2. Update, scan and delete(remove)
3. Remove all malicious virus, spyware, adware and unused registry.
4. Then, start the defragment.
After the defragment, check for services that is running background. Stop all unused services, you may check through Microsoft website to determine whether the service is important or not.
Enjoy a refreshed computer!