Thanks for reply Warwaste...

I am running Windows XP on all 3 Pc's. They are connecting to the router through their own wireless devices, and I have file and print sharing protocol installed on all 3 Pc's.

Make sure all your machines are getting their IP address from the router (set that up under TCP/IP protocol).
They all are, getting 192.168.1.100 then 192.168.1.101 etc

-Check the settings on your router, make sure that the time to live of an assigned IP address is round about 24hours is up to you.
Ok will check this setting

-Make sure they are all set to go out through the router to the net.
Yes they all are.

-Try turning the firewall off on the router.
Will try this and see if it makes a difference

-Check that no firewall software on the computers is blocking packets from the router. (ICMP packets seem to be getting through but nothing else)
They all have SP2 installed, do you think I should turn off the firewall on SP2?

-Try turning off the wifi part of the router.
Not sure what you mean here

-Check what other network services you have running on your machines.
Like what? i have a FTP turned on...

-If all else fails update the firmware on the router.
Just updated the firmware on it.

They reckon a lot of people are getting problems with the WAG54G Linksys which is not good. Someone mentioned to me to see if there was a setting enabled called AP Isolation, which I was told to turn off.

I'll let you know how I get on.