The current iteration release of NUnit 2.2.2 should work fine with assemblies build in Visual Studio .NET 2005. Nevertheless some problems occur after installing Visual Studio .NET 2005 Beta 2. Check out the installation folder of NUnit, usually C:\Program Files\NUnit 2.2.2\bin. Here you should modify the file nunit-gui.exe.config.

<startup>
  <supportedRuntime version="v2.0.50215" />
  <supportedRuntime version="v1.1.4322" />
  <supportedRuntime version="v1.0.3705" />
  <requiredRuntime version="v1.0.3705" />
</startup>

The version of .NET 2.0 on your system is slightly different due to the installation of Visual Studio .NET Beta 2.