Your ISP may be assigning you a particular IP address. Sometimes this is done according to your MAC address. Your best bet is to simply call your ISP and ask them! If they are some big corporation, you'll probably have no luck with that.
Click Start, Run, type "cmd"
From the command line, type "ipconfig /all"
This will give you all of your network settings. Take note of that. Go to your network connections, and click Properties of your Local Area Network. Then go to the properties of your TCP/IP. Manually configure the connection using those settings you just found in the command line, but use a different IP adress (change the last 3 numbers to be different, like 168 instead of 167 or something).