You can stop a subtree of files from being indexed by going to
Indexing Options (literally type that into the start menu) and hit
Modify. Navigate to and deselect the folder you want not to be indexed.
This should stop it coming up in the start menu.
However, if you want it to not show up in search results at all (i.e. in explorer, actually searching on a drive) you may need to do something more intrusive.
Setting a file or folder as
hidden will hide it from the search results as long as you have
show hidden files disabled. You can do this by following this tutorial.
To view the files again just re-enable the
show hidden files (remembering to again disable when you're done accessing them).