Hi impinc are you sorted yet?
I have had problems in the past with redundant registry entries (from failed installs) stopping programs from being reinstalled. Now although I will have a go at fixing most PC related problems (sometimes with the help of the wonderful Google - ahem!!) the registry can be dangerous if you are not sure what you are doing. I have been talked through registry fixes before that involved amending the registry. But this time I went back to a previous restore point on my Win7 64bit PC then tried to install again.
My problem was compounded I believe because of issues I am having with my Nvidia GTX460 gfx card and the 285.XX drivers it's using (being crappy). They keep making my system hang and I keep getting black screens followed by a kernal error message about display driver has stopped responding etc...
I had 2 installations fail mid install due to this and the only way I could then get the 0.75 version to go on was to go back to a previous restore point. The second time this happened to me I also rolled my driver back to version 275.XX which seems to be more stable on my system. After I had rolled it back I put on 0.75 and patched 0.76 & 0.77 without any problems!
PC's can be real buggers at times like this when everyone else is running things ok, and nearly every time it can come down to something specific to your own PC. I'm not saying yours will be the same as mine but are you having any other PC issues at the moment? I can often tell something is not quite right when silly little things start to happen, like not shutting down or starting up from time to time. I think most of my problems over the years have only been down to Nvidia drivers or on a rare occassion when a hard drive is on its way out.
Unfortunately one time when removing my Nvidia drivers to reinstall (on the advice of an Nvidia techie guy) it stopped me from being able to use my mouse or keyboard (even with wired ones). A friend helped me find out that this was a rare thing with Nvidia drivers and my Striker Extreme Mobo!!
Sorry for the ramble, but maybe it can help you identify your issue.
Cheers,
Andy.