I'm running Windows 7 x64, with version 1.1.3 (though this started to happen a couple of versions ago). My problem is that when I'm watching a video (this doesn't happen with audio), the UI responds extremely slowly. The buttons don't turn blue when I hover over them, and clicking them only takes effect a variable number of seconds later. The slider, menus, right click menu and hotkeys all behave like this. Plus, the movie's elapsed time is updated each about 6 seconds. Also, once in a long while the video disappears for a split second and whatever window is behind it is displayed.
Just in case, I tried running an antivirus and it found a couple of things, but it still works like this. I also tried uninstalling and reinstalling, to no avail. Like I said before, music works fine. This happens even without anything running in the background.
I have an AMD Triple core Phenom 8450, 4 GB of RAM and a Nvidia GeForce 8400 GS. A couple of months ago I had a different processor. Now that I think about it, it may have been around that time that VLC started misbehaving, though I can't be sure. Windows on the whole is working pretty well. It's a bit sluggish now and then, but usually everything's fine.