Run yourself a list of what's in tblAssetCustom.
SELECT Distinct Top 1000000
  tblAssetCustom.Manufacturer,
  tblAssetCustom.Model
FROM
  tblAssetCustom
You should be able to use what you find in there to filter on the manufacturer and/or model. E.g. with my inventory, I can include 
  AND (tblAssetCustom.Manufacturer Not Like 'VMWare%')
  AND (tblAssetCustom.Model Not Like 'VMWare%')
  AND (tblAssetCustom.Model <> 'Virtual Machine')
 in my WHERE clause. (I could get away with just checking the model, but I like to cover all my bases.)