Could not launch the product because the specified workspace cannot be created. - ERROR msg

Ivan's solution is better. It is not a good idea to share a workspace folder across operating systems. The root of workspace folder contains .metadata folder where iTest stores files for its proper functioning and some of these are per operating system. You need to create different workspaces for different operating systems.

The correct way to share project files across operating systems is to store the data in a Source Control repository and have a copy of the projects on each one of your machines.

Another sub-optimal way is to create a local workspace folder and then import projects from a shared location. In this case, you can choose to copy the projects or work on projects on the remote shared location. Copying will cause you trouble because after you make your changes, you will have to copy the changes back to shared location (I am assuming they were in shared location because you were sharing these projects across users) and that can cause conflict among users if multiple users are modifying the files at the same time. Sharing the projects will again cause the problem if multiple users are modifying these files at the same time.

The best workflow is to really use a Source Control system for sharing. Then you get  a local copy per operating system and when you are done modifying, you can check your changes into the source control system. You can then checkout changes on another computer and continue working from there.

I think Hietran's problem is that iTest is using the workspace as a default workspace and so he won't be able to switch the workspace. You can try either remove the configurations so as to let iTest ask you the workspace (the configuration files are in your user_home\.iTestConfiguration_xx where xx is the version number; or you can try to re-install iTest and choose a different and accessible path as the default workspace.