Macro only runs on my computer

I wrote an Excel macro for use by others at work. It runs just fine on my computer, but when someone else tries to run it on their computer, they get error messages. Is there something that I am doing, or something that I can do to make it usable by anyone?

Which version of Excel are you using, and which version are the others using?

What's the extension of the file? .xlsx, .xlsm or .xls?

What's the error message? Is it anything to do with macros being disabled?

If the users are using versions of Excel pre-2007, Is there security setting set to high rather than medium? (Tools --> Macro -> Security)

Rgds

We are all using Excel 2003.
The macro security is on the right settings, and the error message that they keep getting is 'Subscript out of Range'.
I don't get it when I run it, but it happens when someone else does.
The file extension is .xls.

If they press Debug when the error occurs, what line is highlighted in the code?

Does it work if you change the code to:

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Thanks a lot. That was it!