Complete Manual Steam Uninstall
I ran into a problem awhile ago involving Steam. I always had it installed on my external hard drive, which is usually connected to my laptop, to save space on my boot drive. I then took my trip to Ecuador, leaving my external drive at home. Because Steam was installed on a currently non existent drive, it wouldn't run (obviously), but more importantly it also refused to uninstall. And last but not least, after downloading a new install client, it also wouldn't let me install it anew on my internal drive.
This could be a massive problem for someone with a failed external drive, etc., so here is my solution. After deleting every last Steam involved file that I could find on my computer and getting the same results as before, I realized the problem must be with registry entries made by Steam. And it was. Luckily Valve provides an amazing guide on how to delete these entries using regedit. Here's the link:
http://supportwiki.steampowered.com/wiki/Uninstalling_Steam
After deleting these entries I was able to use the install client like normal to install Steam on my internal drive and reunite with my long lost games. Hope everything goes as well for you, if not feel free to contact me as always: