Internet connection collapaing !

Hi,

Hoping someone can help me out here.

I've a 120mb broadband but when I connect my router to my PC via the ethernet connection the wireless speeds completly collapse. If I disconnect it from the PC the wireless speeds shoot back up.Tried changing the ethernet cable to no effect. tried rebooting and resetting the modem.Nothing seems to work.

I'm running Windows 7 Ultimate (32bit)

Any advice ?