i assume that there are 2 machines WinXP & Vista and that they are using their own network cards. also, you own and have full access to the router. (some ISPs that provide the router limit the number of PCs connected on the router)
The IP Address is manually assigned on the PC level, not at the router. on the router you can only set the range of IPs and the router IP. since IPs are assigned based on the MAC Address for the network card and since no MAC address is the same there might be something wrong with the range of IPs.
try the following:
1. Go to the router and set the IP lease time to forever.
2. Check the range of IPs that can be asssigned by the router (either 1-253 or 2-254 (254 or 1 used by the router). change them if required.
restart router & PCs and if it still does not work:
3. on each PC go to network properties and on the internet protocol (TCP/IP) properties manually set the IP for the PC. then restart.
Check your router. normally there should be an area with the list of PCs and the IPs assigned to them. Check this also.