I have a Windows XP VM that is stuck in an activation loop. I need to be able to start XP in safe mode to fix it, but I can't figure out how. The VM starts with the XP loading screen, so it's too late to press F8.

This link might be of some help to your problem:  https://web.archive.org/web/20060324053416/http://support.microsoft.com/kb/239780/

The link references Windows Server 2003, but it will work for XP according to this source:  https://communities.vmware.com/thread/263191

 

Now if you're using VNC as the graphics card, you can start the VM and launch the window; keep the VNC Window active and then in the webgui reset your VM.  On that second boot (from the reset) you should see the VM post, and be able to hit F8.