As of Unity 4.2.1 this bug is not fixed yet. Very annoying.
Whenever I try to build web player project or import resources or try to open new project.Unity complains and quits. Is there any solution for this problem?
Just need to restart your pc
Finally I found that Unity is conflicting with perforce explorer integration.
Fixed by removing C:\Program Files\Perforce\p4exp.dll and restarting explorer
This happens quite a lot.
Restarting Unity can fix this (it just did).
Sometimes a full reboot is required. I don’t know what causes this.
[25652-unity+changed+paths.jpg|25652]