Your router is not configured properly.
Contact your DSL tech help.
Try connecting only one computer to your router at a time and observe the results.
Check your IP adresses
if xp
Start > run > type CMD
type "Pconfig" at command prompt.
writ doen the results and compare on both computers.
IP addresses must be diffferent.

What is your operating system ?
Decribe your computer. Brand, Speed, How much RAM ?
Need more info and history about your computer and the problem.