From Autodesk.... I forgot I solved this once in the past.
Sorry guys! Here is the solution:
FS is defined in the express tools. If you are absolutely sure you would
like to undefine the FS shortcut to FASTSEL, comment out the corresponding
lines in c:\Program Files\AutoCAD 2006\express\acetauto.lsp
Replace:
(acet-autoload2 '("fastsel" "fs" nil "fastselect" 2))
With:
;(acet-autoload2 '("fastsel" "fs" nil "fastselect" 2))
Note: This will break the Fast Select toolbar button in the Express Tools
toolbars. To fix this, replace the FastSelect command macro in the CUI
editor ('FS) with ('FASTSEL).