The only situations I can think of, where Windows 7 would prevent you from installing files or programs to a directory, is when you're not logged in with an administrator account, and/or deprived yourself of certain access rights, most likely via the built-in "User Account Control system" (which hopefully I've translated correctly here, I'm not quite sure about its name in the english windows version).
In Windows7, The UAC has been slightly modified and extended, compared to the old infamous version introduced with Vista. Most importantly, at its default setting it no longer keeps alerting users with repeated questions and dialog screens - that's generally an advantage for day-to-day tasks, but you might not even know you have it activated, or at which level.
For both cses, go to your system's control panel, head to the "user accounts" page and check if your account is actually an administrator, and check the UAC's level (there should be 4 possible levels, ranging from "always alert me" to basically "just shut up").
If all else fails try these:
1) Execute the Plugin Manager with administrator priviledges manually (right click the executable file, select "execute as administrator").
2) Open up your Windows Explorer and check if there's a little "Lock"-type icon next to the program files folder. If there is one, try entering the folder and you will probably be asked if you actually want to proceed. Don't ask me what kind of security that is supposed to be, but you can answer "Yes" there anyway. You'll have to answer "yes" most likely once per session, in other words everytime you reboot your computer, unless you change the relevant security settings. After that you should be able to install or copy your desired files into the folder.
It's actually the same with trying to access other users' "My Documents" folders on your computer.
3) Install the game and / or plugin manager to some place else, Windows7 only protects its system directories, the "User" and "Program files" folders this way. Well, actually also folders like "MSOCache" and "Recycled", but you're not gonna want to be using those for anything valuable anyways.

Hope this helps!