Export your level to a .MAP (in Hammer), and open this new file up with a text editor (e.g. Notepad). At the top there should be a list of WADs that your map will be compiled with; if this one is listed delete it, open up the .MAP in Hammer and recompile (keep your .RMF though, in case there are problems loading up the map correctly).
If this fixes the problem, great! If not, then there is a problem with your compilers and paths to Steam, in which case you need to make sure you're not compiling with the original tools (qcsg, qbsp, vis, qrad), and use WADs in Hammer that aren't in your Steam directory (copy them to another folder- e.g. c:temp - but obviously make sure that the originals are still there in the half-life directory somewhere).
On another note, my Steam directories doesn't have an @ symbol in it- why do some people have one? Is it from 'old' versions of Steam? Is it be possible to rename the direcory, removing the @ symbol, and still have Steam work?