So are the guest additions working after you reinstalled them? Do dynamic resizing of the guest and mouse integration work? And does the tray icon show the correct version (1.6.4)?
Well, parts of the guest additions seem to be working. Mouse integration is working, and the guest machine seems to be using the Virtualbox video driver. But dynamic re-sizing doesn't work. I'm not seeing the system tray icon either.
The only advise I can give you is to completely uninstall the 1.6.4 additions and install them again. Maybe some file was not properly installed, there are some problems we are currently trying to debug regarding the WinXP guest installer. Apart from this, seamless on WinXP guests should work with 1.6.4.
At this point I have some reason to believe that it may be related to permissions on the XP guest, because I had another guest on another Linux host start doing the same thing after joining a domain. I've also noticed that the guest additions service refuses to start.