How to completely uninstall and remove QGIS 2.8.1 on Windows Vista?

You can make sure all folders in your hard drive and your .qgis2 folder (C:/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!

There are comprehensive uninstall programs available which do thorough searches and removal for all leftover files and registry values which should solve the problem. An example is Revo Uninstaller.