Opening workbook in user-defined folder
 

I've written a spreadsheet to be used by multiple people. It contains
several VLOOKUP functions going to another workbook. The formula in Excel
specifies the folder where I created the two files. But won't be where each
user puts the files on his/her computer.
I'd like to set up the formula so that, as long as the two workbooks are in
the same folder, the VLOOKUP function will find the right workbook. I
suspect that can be done with a 'period' or some such, but I'm totally
flummoxed. I'm setting up a small macro to run when this workbook opens to
automatically open the other one to get the needed data.

Thanks in advance for any help.