/renew doesn't work in many cases -Here is the easiest way of doing it
log into your modem at:
192.168.0.1
or
192.168.1.1 (paste into url window and click "go")
if it asks you a password and you do not know it then just google
"default password (model of modem)" and you should find it
Once logged in check around until you find the "reboot" button (mine is under "utilities") bookmark the exact page and now you have direct acess to power-OFF/power-ON your modem, much easier than pulling the power plug which eventually breaks the plug after doing it a thousand times (like i did to mine)
This only works if your isp
re-assigns but some isp's take as much as 12-24 hours in power-OFF mode to do that and some don't assign new ip's at all.
Note: this only works if you have already setup dynamic ip adress through network connections so if you have not done that, here's a nice tutorial:
http://portforward.com/networking/static-xp.htm