Delete all personal files , remove any programs you may have installed except operating system, then do a defrag. That will clear up the unused space but every thing will be still there if some one is really determined to get it..
There are tools that will write 0s and 1s (literaly) to the unused space, then 1s & 0s. after several runs, the data will be gone to all but really specialised techniques.
If you want, you can create a load of dummy files to copy to the hard drive, delete them and reload again a number of times. This will confuse the File AAllocation Table, and make it much more difficult to recover any data.
Plan B, remove the hard drive, replace with another unit and start from scratch. Take original HD, take apart, take an angle grinder to the disc surface. Throw all the parts in the bin, no one can get at the info now.
