Check out this post over at JTBWorld.
http://blog.jtbworld.com/2008/09/clear-sticky-sheet-sets-from-recent.htmlThe registry location
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R17.2\ACAD-7001:409\Recent File List
specified is for Autocad 2009 with the following being for Civil 3d 2011
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R18.1\ACAD-9000:409\Recent File List
Other versions use similar locations, just the "R18.1" and "ACAD-9000:409" parts change depending on the autocad version and whether you are using civil 3d, autocad or some other vertical.