If you're having touble getting NUnit to run on Windows 64-bit, you may want to try running <NUnit Root Folder>\bin\net-2.0\nunit-x86.exe, instead of nunit.exe.
NUnit Root Folder is the folder where you installed NUnit. If you're having trouble finding it, check your NUnit shortcut (on Windows Start/Orb menu), or search you machine for "nunit.exe". These two executables are located on the same folder
In these cases, NUnit typically emits the following exception:
System.BadImageFormatException : Could not load file or assembly '<Your-Assembly-name>, Version=<Your-Version>, Culture=<Your-Culture>, PublicKeyToken=<Your-Token>' or one of its dependencies. An attempt was made to load a program with an incorrect format.
NUnit version: 2.5.10