hmm. wide open problem, and I don't know how linux familiar you are, I'm only OK, but here goes some beginner suggestions ....
1) Is your ethernet card up? mandrake control center, / network, look in there.
2) Can you ping yourself? Are you getting an IP by way of DHCP form the router?
3) Can you ping outside, but just not browse? Try ping yahoo.com, if not work, try ping 66.218.71.198 (its yahoo too).
These are just trying to narrow down where you are having problems, for example, if ping 66.218.71.198 works, but ping yahoo.com does NOT work, then its a DNS issue, and we can work from there. If you're not getting an IP address, then its a DHCP issue, and we can work from there.
I mean, there could be a bunch of different possibilities. Maybe the new install, you selected server install and it setup a DHCPD server on your linux box, that then conflicts with the DHCPD server on the router .....
4) type
#service -s
and see if you see dhcpd running.
Any other info you can give would be helpful.
Good luck!