Won't be usable to them without an OS installed. One method which maintains the current OS and all updates is to create a new Admin account. You don't need to put in a password for now. Then, delete your personal admin and any user accounts you created. Also, delete any personal files, and uninstall any programs you don't want to pass on. Also, make sure to uninstall any third party browsers and profile folders, especially if you used the auto password function. I would also delete Windows Page File which acts as virtual memory.
Finally, and most important, follow up with an erase of "free" space on your hard drive for 1-3 passes at most. You can use a free app such as CCleaner.
For all practical purposes, this will make your personal info unrecoverable. Some data is maintained in the registry, but more application and hardware related.
The important detail is to re-write deleted files by erasing the free space. Reformatting, or even repartitioning your hard drive doesn't get rid of all your personal files.
As a test, I once repartitioned/reformatted a hard drive five times, and was still able to recover 70% of files using deep forensic scanning with freely available apps. Which is why you need to re-write the free space to make files unrecoverable.