you need to turn the modem off, unplug the network cable from the PC, plug the network cable into the lappy (while it is switched off), then turn on the modem & leave it until the lights stabilise (about 30 secs), then boot the lappy & it should pick up an IP.
There used to be a limit on the number of times a day you could do this, I'm not sure if it still applies.
<edit> looks like lots of us wanted to help with this one
