I do have admin rights. (I am the sole user. I have changed things where I get a warning "this will affect all users")
I am not quite sure what you mean by "writable", but I did not make any adjustments that would result in it being impossible to make/alter files in the BD directory. I was also able to run the patch, so I'm quite sure that the directory is not protected.
I do not have any error of debug file in my BD directory. Maybe it is located elsewhere? (running a search would be pointless, since lots of programs generate those kinds of files)
I have not defragged/scanned lately, but wouldn't I have some sort of problems with my other games/programs too then?
I tend not to have other programs running. Particularely virus scanners, as they can cause lots of problems, and I'm behind a good hardware firewall anyway. (small local network with shared internet access)