I think most other answers are missing the point here..
The asker states that his Local IP address is changing, NOT his public IP, which is a totally different thing. What is happening here is that for some reason, the DHCP server on the router seems to be releasing a different local IP address to the machine from its DHCP address pool, which is no big deal at all, as the NAT function performed by the router will always translate any local IP address it receives into the globally routable, Public IP address (or Internet Address). The NAT function provides additional security for a LAN, and will bind dynamic ports to each "conversation" from the various PCs on the LAN to keep each connection separate, and is needed because local/private IP address cannot be routed through the Internet.
But if your applications are having bother connecting and you really want to stick to the same static local IP, you could log into the routers' web configuration pages, and under LAN Settings, there should be an option to "reserve" the IP address already given to your machine. This can run within a DHCP server for any IP address within its available IP Address pool.
Alternatively, if that is not an option, you could put a static IP on your laptop, but this MUST be outside the DHCP address pool. On the router you are using, the address space reserved is on the main setup page, under the DHCP server settings. To change your laptops DHCP IP to a static IP in Vista do the following:
1. Click Start and Network.
2. Click Network and Sharing Centre, then select Manage network connections from the left hand side.
3. Right-click the Local Area Connection that is associated with the Wireless adapter you are using,
and select the Properties option.
4. In the This connection uses the following items box, highlight Internet Protocol Version 4 (TCP/
IPV4). Click the Properties button.
5. Enter a unique IP address that is not used by any other computer on the network connected to the
Router, i.e outside the DHCP address pool.
6. Enter the Subnet Mask, 255.255.255.0.
7. Enter the Default Gateway, (this is the Router’s default LAN IP address, NOT the Public IP Address).
8. Toward the bottom of the window, select Use the following DNS server addresses, and enter the
Preferred DNS server and Alternative DNS server (already provided by your ISP).
9. Click the OK button in the Internet Protocol version 4 (TCP/IPV4) Properties window. Click the OK
button in the Local Area Connection Properties window.
You say you don't know what service you are using, so Im gonna guess your possibly sharing an internet connection with other people? If you are, that will be the reason that your local IP address will keep changing now and again, as it depends how many other people have logged on before you and already got their local IP leased to them from the DHCP server on the router. Hope this helps!
For people mentioning Dynamic Public IP Addressing, no Internet Service Provider will provide you with more than one Dynamic Public IP address unless you specifically ask for it and will usually pay extra for. And even then you will be restricted to a handful of public IP's to play with. IPv4 Addresses are already nearing the end of their allocation space, so it is only ISP's who pay their regional internet authority for extra public IP addresses that get them, and they will always pass on that extra cost to the customer.