Hi
Office 2007 has IFERROR which would be easier to use.
As ever with Excel, there are various ways to do things. One way that should get what you want is:
=IF(ISNA(VLOOKUP([Cell with Serial Number],[Range of serials numbers to check],1,FALSE)),"Not there!", (VLOOKUP([Cell with Serial Number],[Range of serials numbers to check],1,FALSE)))
so if the first range of serial numbers runs from cell A1 and the second range is on sheet 2 from A1, the formula in B1 of the first sheet would be:
=IF(ISNA(VLOOKUP(A1,Sheet2!$A:$A,1,FALSE)),"Not there!", (VLOOKUP(A1,Sheet2!$A:$A,1,FALSE)))
Replicate the formula down column B.
The result is that where the serial number exists, it is shown in column B (as a check). If it doesn't exist, you'll see "Not there!"