Further to Todd's suggestion and just in case you don't have that but are using xmod files (which the Windows search facility seems to not search by content, unlike mod files) I know of a nice little utility that might help.
If you download it (free) and look at the Tools > Options section it allows you to specify where you want to search in a couple of ways.
For example you could specify that you want to look for files with names including .xmod. Then drag and drop your C: drive (from My Computer for example) into the 'File' sectgion of the window and AF9 will list all the xmod files on C: rather quickly.
If you then specify the extension ".xls" in the 'Find What' box (as Asc) and run the Count you will quickly see which files have the string .xls in them and how many times it occurs.
You can then specify what you want to search for in the files selected AND what to change them to. (Or not if you just want to see what is there that matches). If you change you can also save the file with a new name and at a new location if you wish.
It's up to you to test it to your satisfaction of course but I have a copy of version 1.2 from about 2004 which I use occasionally and have not had any problems.
The Monarch Utility is really the way to go - but this little tool may just be useful somewhere as well.