When I right-click on a .DOC file and select ‘Open With’ there are many different programs listed. I would like to remove some of these programs, but cannot find how this can be done. Also, how can I add new programs to the Open With list? The computer is running Windows XP.

You are quite correct that it is difficult to remove entries from the ‘Open With’ list for a particular file type. To do this, we will need to go into the Windows Registry. Go to the ‘Start’ menu > ‘Run’ then type ‘regedit’ (without the quotes) and click OK. In the Registry Editor window, navigate to:
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionExplorerFileExts. This should list most of the registered file extensions on the computer. Find the key (folder) named ‘.doc’ and expand the key. You should see a sub-key named ‘OpenWithList’. Select this sub-key, and in the right-hand pane the programs in the Open With list for this file type will appear. To remove a program from the list, select the entry then right-click on the selection and click ‘Delete’. The program will no longer appear in the Open With menu for that file type. Once you are finished, close the Registry Editor.
Adding a program to the Open With list is a much easier process, and you just need to open the file with a program and that program will automatically be added to the list. To do this, right-click on a file with the relevant file type (such as .DOC) and select ‘Open With’ > ‘Choose Program’. Select the program from the list, and click OK. The file will be opened in the selected program, and the program will be added to the Open With list for that file type.