Getting BadImageFormatException errors when trying to use NUnit 2.2 with VS2005 Beta 2? There are several spots with the answer on the web. I found my solution at IdentityCrisis.
Edit the nunit-gui.exe.config file and add support for the latest framework. Move it to the top of the list of supported runtimes or it won't fix the issue.
<supportedRuntime version="v2.0.50215" />
<supportedRuntime version="v1.1.4322" />
<supportedRuntime version="v1.0.3705" />
<requiredRuntime version="v1.0.3705" />