What about the temp files, cache, internet history?
Disc defrag will straighten everything up but you need to get rid of the junk first to make it really effective.
If you don't mind I will give you some guidance first.
You have to check the programs that autorun in the background that you may not know about. These can use a lot of system resources thus slowing the system down even more. Download Autoruns from the link in my signature below:
1: Extract the Autoruns Zip file contents to a folder.
2: Double-click the "Autoruns.exe".
3: Click on the "Everything" tab
4: Remove any entries that mention "File Not Found"
by right-clicking the entry and select Delete.
5: Go to File
then to Export As
in some versions.
6: Save AutoRuns.txt file to known location like your Desktop
7: Attach to your next reply.