When it stops can you still get a leased address from the router? When it stops try issuing the following commands in a Windows XP command line:
ipconfig /release
then:
ipconfig /renew
This will release the IP address from the router and then it will ask the router for a new address.
If you get a new address can you browse the network, ie. Can you ping each computer? If you can the internal network is probably still working but it may be something with the router.
If the network seems to be working try renewing the address the router has on its WAN port. See if that fixes the problems.
Reid