Steam Incomplete installation(55) Warhammer 40k:space marine demo?

I download the warhammer 40k:Space Marine demo from steam and when it finished it said incomplete instalation(55) and on steam support i was told that it had to do with avast antivirus interfering with it or something. is there any way to fix this without getting rid of avast?Steam Incomplete installation(55) Warhammer 40k:space marine demo?
No, but there are many other free anti virus programmes out there. Try one that will not interfere.

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