You can make sure all folders in your hard drive and your .qgis2.qgis2
folder (C:\Users\You.qgis2C:/Users/You/.qgis2
) are deleted. You could also delete the Registry entries for QGIS by running the regedit program and searching for QGIS:
Hopefully when you restart your computer, you will be able to reinstall QGIS again.
Hope this helps!