Another option: fdisk and format the hdd and reinstall windows.
Formatting will not securely delete data - only make the space available when needed.
After that it depends on how valuable you think your data is / how paranoid you are about security and privacy. :-}
So you could get a free program like eraser and use it to erase the unused space on the hdd. Even use it before formatting to delete your data and then reinstall Windows.