Looking back in the logs, this has been happening intermittently for a long time. Native os is Win 7 64 bit, I run the game in compat mode for Win XP SP 3. I didn't think it would run unless it was in compat mode, guess I'll try and see what happens. I do check regularly for virus's malware etc, The file name in the folder is lower case and the right length, and there is another in AppPatch64 that's different. Any doubt, here's the locations (note the .old stuff shouldn't make any difference, I'm just too lazy to delete that folder).
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Users>cd\
C:\>dir AcXtrnal.dll /s /p
Volume in drive C has no label.
Volume Serial Number is 0CED-8EA0
Directory of C:\Windows\AppPatch
07/13/2009 09:14 PM 211,968 AcXtrnal.dll
1 File(s) 211,968 bytes
Directory of C:\Windows\AppPatch\AppPatch64
07/13/2009 09:40 PM 135,168 AcXtrnal.dll
1 File(s) 135,168 bytes
Directory of C:\Windows\winsxs\amd64_microsoft-windows-a..ence-mitigations-c5_31bf3856ad364e35_6.1.7601.17514_none_6a4c3ec108e85b6a
07/13/2009 09:40 PM 135,168 AcXtrnal.dll
1 File(s) 135,168 bytes
Directory of C:\Windows\winsxs\wow64_microsoft-windows-a..ence-mitigations-c5_31bf3856ad364e35_6.1.7601.17514_none_74a0e9133d491d65
07/13/2009 09:14 PM 211,968 AcXtrnal.dll
1 File(s) 211,968 bytes
Press any key to continue . . .
Directory of C:\Windows.old\Windows\AppPatch
04/14/2008 06:41 AM 116,224 AcXtrnal.dll
1 File(s) 116,224 bytes
Directory of C:\Windows.old\Windows\system32\dllcache
04/14/2008 06:41 AM 116,224 acxtrnal.dll
1 File(s) 116,224 bytes
Total Files Listed:
6 File(s) 926,720 bytes
0 Dir(s) 40,205,828,096 bytes free
C:\>