Spent a couple of hours yesterday trying to fix this. I didn't. I have made sure the files are not read only, took possession of the FF folder, set it up to run under every compatibility mode they had (it worked under Windows 7, but FFv3R runs under the compatibility mode as WXP Service Pack 2 just fine). I've used a program manager to watch as the FF program and a .dll load, then are killed. I believe it is a security issue, and despite being an Administrator, it doesn't believe that I should run the program.
The annoying thing is as time goes by (and I mean weeks), other stuff starts acting funny. At first, I was able to convert FF1 .nifs to FFv3R easily; a few weeks back, I had to create a folder just for conversion,and it took an hour to set up the permissions so that it will let me convert. I suspect that eventually that will stop after a later update.I am going to try again to put it on the C drive, but am open to any and all suggestions.