I am just about to enter the lair of the very final baddie.
BUT, when I try to enter, the game beginns to autosave as usual... then freezes. And nothing more happens. ´Not even Alt + F4 or Ctrl + Alt + Del works. I have to turn off my computer with the powerswitch.
This is very anoying (hard to believe hu). Does any of you know how to solve this? I have restarted the game several times with the same result, I installed the latest patch just some hour ago.
And I DO have TotSC installed.

Check out the "Gatekeep" topic. You may need the Gatekeeper program to repair a damaged save. I don't remember where to download it but a forum search should find it.

Ooookaaay. I tried to repair the saved game. No improvement.
I tried to uninstall and then reinstall the intire game. Improvment. Now the game auto-saves, and locks down forst when I come to the load-screen.

Anyone knows what to do?

One thing to try is to make sure you've got Disc 4 in the drive. There's a movie right before the final baddie's lair, and sometimes when you've got the wrong disc in when it's time for a movie, the whole computer locks up. This is true for BG2 as well, I find.

If the movie doesn't start proper from the CD, then another option (if you have the HD-space available) is o install everthing on your HD (this takes with TotSC about 2.5 GB, IIRC). This also solved many problems with Planescape.