I've been on the verge of seppuku for an hour now because of problems trying to install Eclipse plugins on Win 7. I use the same automatic install feature I always have, installing from an update site. Everything seems to work fine, but when Eclipse restarts, there's no plugin. It appears in the list of install software, but no plugin. Checking out the eclipse directories, I see that the proper plugin files aren't there. Argh!
Well, it appears that on my machine one needs administrator permission to change the Program Files directory. I'm not sure why it became necessary, but the fix is simple: Run Eclipse as an administrator. To do that, just navigate to the directory, right click on the file, and click "Run as Administrator." You can also set it to run that way permanently by right-clicking on the file, choosing "Properties", then the "Compatibility" tab, and checking "Run this program as an administrator".