Without having to reinstall just stripping out your data and programs could be sufficient if you run CCleaner Disk and Registry cleaners afterwards then use it's Tool to Wipe empty disk space as suggested. That's probably sufficient since it would be nearly impossible to recover data using all but forensics tools.
Another option is running Factory Recovery from its partition or disks which will format over the partition before reinstalling and set PC back to factory condition. You'd not need to do much more than that since it allows the new owner to set it up.
To get the cleanest possible wipe you'd want to run Diskpart Clean ALL Command
but that requires a clean reinstall or running the Factory Recovery disks.