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Descent 3 Error
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Error: Unable to set temporary directory to: "C:\Program Files (x86)\GOG.com\Descent 3\custom\cache"
The directory is in use, please use -tempdir to set a different temp directory
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OK
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Then it won't let me play.

Hm, I can get around the error by deleting the content of that folder, however when I run it it just minimizes and does not play, can't seem to find a way around that yet.
Ah ok got it
Had to disable visual themes and stuff like that.

Gah I am having the same problems...
Asus P5q3 deluxe 2.33 Intel Quad
Radeon 4850x2 Sapphire

Did you ever find a fix for this? I've been getting the same error

Look in the custom\cache folder for a file called .LOCK and delete it. That should remove the error about the temp directory.

go to task manager and find the process MAIN.exe and terminate it, it should be fine after that