Sometimes I'll get that error if some of the DLLs that my plugin depends on can't be found. If you're building with Visual Studio some MS DLLs are required. For .NET 2003 they are mfc71.dll, msvcp71.dll, and msvcr71.dll. These DLLs are shipped with Maya and should be in the Maya bin directory.
.NET 2005, however, has newer versions of the DLLs (with '80' instead of '71' I believe). Since Maya 7.0 was released before .NET 2005, they aren't included in the Maya install by default.
If you think this might be the problem you're encountering you could either manually copy the 80 DLLs to your new machine (the three mentioned above are marked as redistributable by MS so you should legally be able to distribute them with your plugin), or if you could build with .NET 2003. If you don't have .NET 2003 you could download the free .NET 2003 toolkit, as well as .NET framework and Windows platform SDK - those three, free, downloads from MS should be enough to build a plugin using with the .NET 2003 compiler.
If you're already building with .NET 2003, doublecheck that all of the DLLs that your plug-in depends on are in your Path.
Hope this helps.