One of my clients uses ACT! 2007 on XP SP2 with Outlook 2003. Last week, all of a sudden her Outlook hung, and since then she hasn't been able to open up ACT!.
When logging in as a local administrator, ACT doesn't run either. The main window opens (the one with "New database" and "Open database" buttons), you can choose whether to check for updates, but then it just hangs (using 99% of CPU most of the time) and you have to kill ActSage.exe in the Task Manager Process window.
I have tried uninstalling ACT! and MS SQL Express 2005, the .NET Framework 2.0, I've used the Uninstaller app, selective restart, ... the whole procedure as documented here: http://community.act.com/sage/board/message?board.id=ACT&thread.id=4424 but it just doesn't help, still the same problem. Judging from the manual, this should be the moment where the registration/activation window should appear when you've just done a fresh install, I don't know if that has anything to do with it.
I'm pretty desperate, been trying for over a day now, and given that the location is pretty far away from my place, reinstalling the whole computer is not an option unless absolutely necessary.