The description leaves some holes to the story. So have you extended your AD schema so that there is attributes 'Laptop-Serial No' and 'Desktop Serial No' and you ask how to populate values to those attributes or are you asking how to extend the schema to have the two attributes in the first place?
Typically, you want to leverage unused attributes in your current schema, not add specific named attributes. For example, you might look at extendedAttribute# attributes (e.g. extendedAttribute12) and leverage them to hold data you'd like to associate with a user.
I believe those extended attributes are only there if you have installed Exchange. But there are a few others you might be able to steal, if they are unused. You can always leverage the notes field unless it has a specific purpose already, and you gain the ability to access it directly from ADUC without having to use the attribute editor. Extending the schema in a production environment is best left to a qualified AD engineer.