On each PC (assuming they're both running WinXP) open a command prompt and type ipconfig and compare the response. If the "Subnet Mask" are both the same it eliminates one problem

ive lost the other PC now, its all going pete tong

neither PC can see the other now

okay im back to square one where my main PC can see the spare, but not vice versa

TBH I had a similar problem quickly networking two PC's at work. In the end we ran the setup wizard because every setting was correct - but it still refused to work Ran the wizard and it was happy as ....

Like he says - go to a Dos prompt, and type IPCONFIG /ALL
You should see settings for the ip address and the subnet mask.
The IP address will probably be in the range of 169.0.0.0 and the mask should be something like 255.255.0.0 - subnet masks need to match, and whichever numbers on the ip address that would be covered by 255 also need to match - so in this case both IP addys would need to be 169.0 plus a unique number. If this subnet mask is 255.0.0.0 then only the first number needs to match. That way, both machines will be in the same subnet and won't need to use a gateway.

That make sense?
once you know the ip addys of the machines you can also ping one from the other.

make sure your login name and password is the same for both PC's as well. And make sure you add the IP address ranges to zone alarm or any other software firewall you have running.

So what ur saying is i could run 2 computers for exsample, and back up all my docs on the other computer and as i download new files the file will be copyied over the network to the other computer and vice versa?