So far it happened upon returning to the Blackrock Clan mines and Ashbury - the game slows down, the loading wheel shows up and... there we go. If I alt-tab fast enough, I can probably ctrl+alt+del to close Arcanum - otherwise, even the task manager screen is frozen and I have to hit restart (and then cancel) in order to close the game.

I'm assuming this happened because I installed either the unofficial patch or the HQ townships patch, but I'm not sure which, and I'm not sure if they'll stick around when I reinstall the game.

Huh. Just happened in Blackroot - no problems when I got there, then a huge slowdown in the eastern part. I managed to hit a teleport to another location, and the problem was solved - probably because the slowdown did not occur as the game was loading something.

'k, I reinstalled and this is still happening. Help?

Could be related to video card. Try to launch the game in software rendering mode using command line arguments -no3d -doublebuffer as explained here:

http://www.gog.com/forum/arcanum_of_steamworks_and_magick_obscura/arcanum_weird_bug

Tried that, as well as reinstalling the game and deleting everything that seemed to do with the patches - nada.

Just a very wild guess, try disabling your antivirus if you have one.