I presume you are using the Linq To Wmi project at http://linq2wmi.codeplex.com.
If so, then the published version only supports constants and not variables in query expressions. (Your query would end up generating something like ... where Description = 'theDescription' ...
which is unlikely to return any results.)
But if you go to http://linq2wmi.codeplex.com/SourceControl/list/patches you will see that someone has submitted a patch to resolve this issue. It was submitted in 2009 but is never accepted.
There have been a few minor changes to the published source since then, but it should be fairly trivial to merge them.