You can nest any formulas in Excel, so replacing "ERROR"
with another IF(ISERROR(...
is surely possible. Ensure that the formula you want to place is correct by itself and it should work.
=IF(ISERROR(SEARCH("7",[@Caption])), "Windows 7", IF(ISERROR(SEARCH("Windows",[@Caption])), "any other Windows", "Error"))
Note that I added and @
as I think this was missing in your formula!