start>run> cmd>ipconfig/release/all>ipconfig/renew/all. This is the win2K pro command line, XP is different but, I dont have XP handy to get you the line. ipconfig runs under a DOS shell, if nothing else grab a dos prompt somewhere and cd .. back to c:\
See if it reassignes a different IP. For most routers the ip for setup is 192.169.1.1 unless its been changed to 192.168.1.2. Once in the router setup make sure its configured for a static IP for the computer and dynamic for the modem. This way the router keeps the connection alive by swapping IP's as needed. The PC always see's the same IP from the router. If you have mulitble machine, remember the boot of the network running static ips. The router will hand the ip's out as 192.168.1.101, 192.168.1.102 and so on.
Sometimes the modems ip is also 192.168.1.1 and most default pass's for them are
admin
1234
or
admin
admin
These also apply to routers. 
If you want, you can force change the routers setup addy to 192.168.1.2 and leave the modem as 192.168.1.1. I know on sprint dsl, the modem will get irate if you change the ip. 
