I tried adding the control to the Toolbox, browsing it, and this error appears. I had exactly the same error when installing AjaxControlToolKit 1.0. What I did was to download the source code and recompile it.

 

I know that it's impossible for you to give us the source code, but you could try compiling it in a Vista machine to see if that works. If not, then there won't be any solution...

 

Regards,

Francisco