can't create new project

Unity basically restarts when creating a new project or opening another project. On Windows Vista that can result in an application is unresponsive dialog but you can close that dialog (not let it close the app of course). If Unity really doesn't come back, look at the log file of the editor. It's a bit of a hassle to find it under window (it's probably documented somewhere) but for me it's under my home directory, AppData/Local/Unity/Editor. If you're on an apple it's in your home dir /Library/Logs/Unity , look if you can see the reason of failure in it. Guess you've tried different directories and no default assets to try if it works without them?