I am running qwest DSL through a Linksys wrt160N

If you have concrete floor slabs the the wireless signal from the Linksys will have a difficult time penetrating the floor slabs.
The wireless card in the computers will show a low signal strength.
Try altering the Linksys aerial to a horizontal position and also positioning the Linksys to another location.

You could be getting interference form other wireless sources, check the wireless channels of other wireless router/access points in your area. If they are broadcasting on the same channel then configure your Linksys to broadcast on an unused channel or one that has the weakest signal strength.
If none of these methods work then I would suggest you run network cables from the Linksys to upstairs and downstairs and not use the wireless connection.
If network cabling option is not an option then consider extending the wireless range with wireless repeater access points.