.NET.. runtime error "assembly's manifest definition does not match the assembly reference"

.NET.. runtime error "assembly's manifest definition does not match the
assembly reference"

I copied a .NET project over from one computer to another. When I compile
it, it compiles ok, but when it loads in the browser, i get the following
error:
Could not load file or assembly 'AjaxControlToolkit (1410)' or one of its
dependencies. The located assembly's manifest definition does not
match the assembly reference. (Exception from HRESULT: 0x80131040)
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.
Exception Details: System.IO.FileLoadException: Could not load file or
assembly 'AjaxControlToolkit (1410)' or one of its dependencies. The
located assembly's manifest definition does not match the assembly
reference. (Exception from HRESULT: 0x80131040)
I'm not really familar w/ .NET so I'm unsure what other steps I need to
take in order to get this to compile and run successfully.
I checked my REFERENCES .. and I can see the (AjaxControlToolKit.dll).. So
I'm wondering what else I'm missing.