Yeah, it sounds like something's blocking the installation of addons/extensions. It could be some kind of strange windows permissions thing, or (and I think this is more likely) some bit of anti-malware or firewall software, maybe even a sandbox, that is creating your problem. My advice is to try again to shut everything off that could possibly be blocking the installation and try to install the greasemonkey extension (firefox) and/or the icequickstream (chrome).
Ignorance and callousness are not virtues
Check to make sure your scripts directory is read/write (archive - I think windows calls it?). Access the script (in chrome it's with the addons in the wrench, and in firefox go to tools, addons, user scripts), then right-click in a script box anywhere there isn't anything and select to open the containing file. When the file manager opens, navigate backward one level and check the properties on the folder to make sure it's writable. Do that all the way up the tree if you need to (I'm not sure if that's entirely necessary, but better safe and all that). Definitely check all of the firefox/chrome directory tree, but if it's affecting both browsers, it's likely something more than that.
Perhaps you need administrative privileges to install anything?
A bleeding heart is a symbol of holiness, a symbol of compassion, not an epithet
When our discourse sinks to the level of meaningless, pejorative labels and personal name-calling rather than honest reflection and reasoned dialogue, we are no longer a people, but a mob