Windows 7: ethernet cable plugged in drops internet connection

hi,
first of all sky is working ok. i have broadband....it connects to a laptop via wireless...but!

if i plug in the ethernet cable from my desktop, it knocks out the connection!
this pute is a new re-build. all was perfect for the last 2 days [when i assembled it] and was ok this morning. i left the pute for 10 mins, and have had this prob since.

i phoned support, done the tests, and we both put it down to a broke ethernet cable. i had a spare cable [another sky one] in the house which i found later, tried it...no joy.
i then bought a new one...same result!
i have downloaded all new drivers for my mobo, i reset cmos....i re installed windows!!



pute connects to a network [ ethernet port is flashing] but wont to internet.

i enabled DHCP which for a minute, i thought fixed it but the connection still dropped albeit after another minute.

really need some help here please!

regards.
jim.