A change to one of our client's machines caused the "Open with" context menu to disappear. This menu appears when you right click on a document or other data file that's not an executable or shortcut. After checking with Microsoft, we found the solution from MS MVP Daniel Martin:
1. Click Start, All Programs, Accesories, Notepad.
2. Copy and paste this text:
Windows Registry Editor Version 5.00
3. Click File, Save As, name the file "OpenWith.reg" (WITH THE QUOTATION MARKS) and click OK.
4. Now double-click OpenWith.reg, click Continue, Yes, OK, and see if the problem is solved.
This registry addition resolved the issue perfectly. Thanks!