Oracle Windows OPatch Prerequisite check "CheckActiveFilesAndExecutables" (Following files are active) failed.

When you are running Oracle OPatch patches in Windows environment you might get following error:
Running prerequisite checks...
Prerequisite check "CheckActiveFilesAndExecutables" failed.
The details are:

Following files are active :
C:\oracle\product\10.2.0\db_1\bin\oci.dll 
.
.
.

Patching guide says that before patching you should stop all Oracle services along with the "Distributed Transaction Coordinator" service. But sometimes this is not enough and you get "ActiveFiles" error when trying to run OPatch.

Usually problem is Windows Management Instrumentation (WMI) service. Restart this service from Services tool or you can also end process "WMIPrvSE.exe" from Task Manager (it will restart automatically). Then try to run OPatch again.

From following MOS (My Oracle Support) document you can find more help to this error:
Files in Use errors, when applying patches on Windows [ID 418479.1]

There is also Process Explorer tool which you can use to check which process is using certain ddl's:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx