Unity may have gone a little insane for one of various possible reasons. (For me it usually happens when I change the project's name)

It happens to be sometimes but I'm a mac so I don;'t know what insane Unity looks like on a PC.

Try exporting all your assets as a uniytpackage,creating a new project, and importing that unitypackage.

I had something like this. Try turning off virus scanning. If you hit Try Again enough, does it finally work? And yeah, 3.1 might fix it (I haven't seen the problem in a while). If it is the virus scanner, you may be able to exclude folders/files from the scan that interfere with Unity.