If the hash was ok then file should be too
I used to get this on my other comp
archives etc would pass hashes and be free of errors but still fail on extraction sometimes and install too
Never found the cause
Probably something hardware related
You could try asking in this forum JC
http://www.subsim.com/radioroom/forumdisplay.php?f=235
If you do decide to remove SH3 next time you install make a copy of the data folder
That way you can just drop it in you SilentHunterIII folder instead of reinstalling all the time