Same Dll-version Different On Different Computers?

i have a strange problem with a VB.Net Windows Application. On my computer(the development PC) it executes the newest version of the exe and dll's but on the computer of my fellow(he is not a developer and cant debug it) an older version is executed. He has overwritten all files with the new version:

I dont know where to start looking for the reason.

EDIT: the first thing i've done was to restart the pc of my fellow but that was of no use.

When is a dll installed in the global assembly cache? Because i've searched for it in the gac with gacutil.exe but not found:

C:>C:WINDOWSMicrosoft.NETFrameworkv1.1.4322gacutil.exe /l Rule.dll
Microsoft (R) .NET Global Assembly Cache Utility. Version 1.1.4318.0
Copyright (C) Microsoft Corporation 1998-2002. All rights reserved.
The Global Assembly Cache contains the following assemblies:
The cache of ngen files contains the following entries:
Number of items = 0

I've also looked into the assembly folder but not found any of these dll's there.

Additional info: one of the dll's is loaded dynamically in the codebehind:
System.Reflection.Assembly.LoadFrom("Rule.dll")
The others are referenced in the particular project.