Cleanup junk files with tools like CCleaner and System Ninja.
Check for malwares - Roguekiller, adwcleaner, MBAM
Check for fragmentation and defrag if required (Not to be done on SSD)
Remove unnecessary programs from startup
Check that the partitions have at least 20-25% of free space...
Back up your data.
Boot from the USB.
Follow the instructions and choose "Install Now" when prompted.
Choose "Install Windows Only (Advanced)"
Select each partition and delete it. This deletes the files on the partition
When you've finished this, you should be left with "unallocated space"...