Windows 7: Morrowind Plugins won't extract

So I got Windows 7 up and running today and the first game I put on was The Elder Scrolls III: Morrowind. The game itself is fine, it's the plugins/mods that won't work.

If you've played Morrowind, I'm sure your well aware that with very good mods, you can make it look and play so much better. My problem is that all of my mods are in .rar or .7z format and it gives me an error when I extract anything, "Cannot open..."

I have tried both WinRar and 7Zip and both have failed me, any ideas guys?

Thanks!