At the minute I have my Cable Modem (Telewest Blueyonder) plugged into
a Linksys WRT54G which in turn is plugged into my network card in my
PC. Internet works fine on the PC.

I also have a Laptop with a 'Lan Express AS IEEE 802.11g miniPCI
Adapter'. Through this I can get Wireless Internet, which works fine
too.

Both systems are running Windows XP Pro SP2.

My problem is I cannot for the life of me figure out how to get file
sharing working between the two.

I need step by step instructions of how to fix this.

if uare asking how ato shae file between two computers then its too much easy 
 one thing to be noted file canot be shared only folder or drive can be shared 
put all your file u want  to share in one folder and then close the
folder  then rigth clich on the the smae folder icon and click
properties , then click sharing .then tick mark the option "share this
folder on the net work " tehn click ok and colce the opened window  if
u wnat to changet the shared folder form the remote computer then also
click the second option that is "allow the network user to change my
file " in this uway u can acces and change the file frol your remote
combuter

google search: file sharing xp

http://www.practicallynetworked.com/sharing/xp/filesharing.htm

you can also try testing connectivity between the two comps from the ping command.

Thanks for the reply. I have already done this. The shared folders
still aren't visable via the "my network places".

First of all, you need to adjust the settings on your firewall to
allow file and printer sharing between the two computers.  To test and
see if the connections are working, you should ping them.  Go to
Windows Start, click on Run, then type "command" to get a dos window. 
At the prompt, type "ping" + the other computer's IP address (usually
192.168. 1.x).  You should get reply back from the other computer if
the connections are there.  If you do not get any reply back, you need
to run the set up network process again and re-check the firewall
settings.  Let me know if this works for you.