If you've tried an install and it's not worked, or tried an uninstall and it's not worked, or reinstalled a service patch or MCE rollup 2 it can cause .net issues.
Try the dotnetfx_cleanup_tool listed on this page (most of the way down the page down on the right, under useful tools) to remove all traces and try reinstalling .net again with the service packs.
http://blogs.msdn.com/astebner/archi...30/413764.aspx
There's also a lot of other great info there.
Note that there are removal tools for .net 1 and 2.
Hope that helps?