If you are on cable you can always try ipconfig /all in the command prompt
This will then tell you when your IP is up for renewal.
example:
Lease Obtained. . . . . . . . . . : 29 May 2006 09:34:46
Lease Expires . . . . . . . . . . : 30 May 2006 15:11:31
About 4 hours before the renewal time disconnect your modem from the stream
(unplug from cable and power) leave it 24hrs and all should be well, I learnt this from an NTL technician
Hope it helps!!