Hmmm, I'm not sure I'd set up so many fields to use looked up values here. Instead, I'd just place fields from the related table in my parent table's layout...
There is a difference in what happens when you edit data in the related table, so you'd have to decided if that's a better way to go or not.
In any case, you can make your serialNO field use a drop down list that displays both serial number and product name in two columns with the serial number field selected for column 1.
Now you can look for the ProductName in the second column of the drop down list.
Once you get that working, you might consider a more sophisticated approach that would let you pull up products by entering all or part of the product's name in an auto-complete enabled drop down "search" field:
thanks for the quick reply, I'll have a try on the auto complete drop down list later today, which looks glogious thank you!