Before you uninstall an express SKU using the uninstall tool, make sure you register the product. Otherwise, uninstall will hang. The Express product is expecting a activation code, however, since we're uninstalling silently, the UI is suppressed and the process just hangs there.
If you get yourself into this unfortunate state, go to task manager and delete the process.
If it's Web Developer, kill the VWDExpress.exe. (The other express products will look similar to that exe name).
The downside to this is the templates that get installed won't be cleaned up.