See this thread: http://www.lansweeper.com/forum/yaf_postst2718_Report-shows-same-product-key-for-different-Product-types.aspx
Your report will look something like this:
Select tblComputers.Computername, tblSerialnumber.Product,
tblSerialnumber.ProductID, tblSerialnumber.ProductKey
From tblComputers Inner Join
tblSerialnumber On tblComputers.Computername = tblSerialnumber.Computername
Order By tblComputers.Computername
! Don't add the software table to this report.