Replace the relevant bits in your nunit-console.exe.config and nunit-gui.exe.config with this:
<startup>
<supportedruntime version="v2.0.41013" />
<supportedruntime version="v1.1.4322" />
<supportedruntime version="v1.0.3705" />
<requiredruntime version="v2.0.41013" />
</startup>
BTW, the config files live with the .exe files, e.g. in C:\Program Files\NUnit 2.2\bin