workbook links to .xla file on the network

I have few custom functions in the C:/my.xla file. If I add my.xla to the workbook on my local drive and use functions from it everything is fine but the minute I copy workbook to the network (lets say drive F) and try to open it then link to the C:/my.xls automatically changes to F:/my.xla and formulas do not work anymore.
Is there a way to permanently specified the path to my.xla?
Thank you!