You want to expand on the WHERE clause.
To list only Dell machines
WHERE
tblAssets.Assettype = -1
AND Coalesce(tblAssetCustom.Manufacturer, N'Unknown') LIKE 'Dell%'
To list only machines where the model name starts with Latitude
WHERE
tblAssets.Assettype = -1
AND Coalesce(tblAssetCustom.Model, N'Unknown') LIKE 'Latitude%'
Build it up if you want multiple conditions
WHERE
tblAssets.Assettype = -1
AND ( Coalesce(tblAssetCustom.Model, N'Unknown') LIKE 'Latitude%'
OR Coalesce(tblAssetCustom.Model, N'Unknown') LIKE 'Precision%'
)
If you're unfamiliar with using LIKE, the percent symbol matches anything, so 'Blah%' means "starts with 'Blah'", '%Blah' means "ends with 'Blah'" and '%Blah%' means "contains 'Blah'".