Ethernet - LAN card installation problems

My ME computer does not recognize the LAN card installed to provide with
ethernet access to connect to internet via dsl (AT&T-YAHOO)). I'm using slot
#3 and am certain that is properly inserted. When I try to make the system
look for it, it does not find it, so I force it from a list of manufs and
models (Linksys - LNE100TX(v5)). Although it goes through requesting drivers
and files, which I provide both from the CD ROM and later from a download
directly fm LKSS, and it concludes with a "completed installation", when I
restart there are no signs of it. I got so frustrated that bought a USB LAN
to Ethernet device/adapter but this also does not work. Although it
recognizes it, somehow it can not find the drivers and/or files supplied in
the CD ROM.
Anyway, to get to the point/q, what can one do to make one's system
recognize an installed LAN card in the system?
Thx much.

Ro