Excel 2003

I have a spreadsheet function that looks something like this:

=IF(OR(RangeA=0,RangeA=""),"",IF(RangeA-15000<=0,"",RangeA-15000))

I need to add more conditions, so I'm trying to write a user-defined
VBA function. What code (if any) can be included in such a function
to get the first result shown above, i.e., a blank cell (symbolized by
"" in a worksheet function) under certain conditions?

Thanks.