Google "win vista configure fixed ip address" w/o quotes.
You must know the first three "quads" or a.b.c.x ("abc") part of the router's IP address. The x value you choose can range from 1 --> 254 but cannot be that of the router's IP, or within the DHCP range of the router. If the latter, an addressing conflict will occur between your chosen fixed IP address and that same value eventually dynamically assigned by the router when requested by a client. If the former, your network is dead in the water.
I'm assuming you know how to use the web menu of your router or AP to determine the values you need. There are other means as well, for example entering "ipconfig /all" in a black Command window in a windows pc already connected wirelessly to the network.
The abc-part (sub-net) should be that of the router's IP address. The mask will be 255.255.255.0. Use the router's IP address for the gateway value, and try it also for the DNS.
If by chance the router's IP is 10.x.x.x or 172.x.x.x, report back.