Windows 7 themes

Ok, I have a theme on windows 7 set up with about 200 backgrounds on rotation. When I restart my PC, it decides it's going to pick one of those backgrounds at random (with the same colour/transparency taskbar) and create a new theme called "custom theme". To get the backgrounds to start rotating again I have to go back into the theme selection menu and select my saved theme.

Why is it doing this and how do I stop it? I've tried googling but the only results seem to be for XP! +rep for helpful answers

The best place to go would be www.sevenforums.com

They will help you out instantly.