We struggled a lot with upgrading WordPress plugins on my IIS 7.5 server. It just failed with an error telling that you it could not delete the old files, than the plugin gets disabled and you loose permission to the pluginfolder. When I Googled the problem I found posts suggestion you to add IUSR user which did not solve my problem. Finally I found a blog that explains the issue and suggest you to a upgraded version of the wincache PHP extension.
I downloaded the lastest version which currently is wincache-1.2.614-dev-5.2-nts-vc6-x86.exe from http://sourceforge.net/projects/wincache/files/development/
Unpacked the package and copy/overwrite the php_wincache.dll file into the PHP extensions folder. Typically this folder is called “ext” and it is located in the same folder with all PHP binary files. For example:
Recycle your IIS Application Pools for PHP to pick up the configuration changes and your done.