You should be able to go to the linksys page and download appropriate drivers/manuals.
More than likely you will hook up the Wireless router to the DSL modem downstairs and then hook the downstairs computer up directly to the router. Most wireless routers also have a couple wired ports for this purpose. It will facilitate the connection for your wired computer as well as wireless. Once the downstairs computer is hooked up open a web browser and connect to the wireless router via IP address. Typical addresses are 192.168.0.1 and 192.168.1.1. If it asks for id/pw try admin/admin, admin with no password or admin/password. If none of this works get a copy of the manual as it should tell you all the addresses and passwords. Once you get in you can configure DNS info, security, etc...

