Many thanks Kryten and jb99 for your replies - much appreciated...

Strangely enough, following my previous post last night, in desperation I searched my hard-drive for 'WinX' (using Agent Ransack). To my surprise the search revealed two folders both named WinX. One was empty but the other contained 3 files named - Group1, Group2 & Group3. I copied these three files and pasted them into the empty WinX folder. Lo and behold, on restarting my machine, the right click on the start menu was up and running again. Amazingly simple considering I'd spent a month seeking a fix!