Connecting 2 computers one with ethernet one WiFi via Thomson TG585?

[#1]

Hi,
I have a desktop computer connected by ethernet cable to the Thomson TG585 router  then to the internet; I also have a laptop connected by WiFi  to the Thomson TG585 router  then to the internet. Both work fine and I don't want to mess that up!
What I want to do is add a connection between the laptop and the desktop computers (so I can transfer files) via the Thomson router without adding any other connections and without disturbing the existing connections. Is that possible?
If it is possible, can someone point me to instructions on how to do it please?
Thanks in advance,
Bill

[#2]

Yes it is "easy" you don't say what OS you are using so I will be generic
On pc1 you need to Share a folder (Right click and choose Share/sharing options/etc)
On pc2 you need to browse the Local Network - choose pc1 and the share you set up above.
Then dependant on the rights to have set on the share (ro rw etc) will restrict/not what you can do with it.
If you can't "see" the other pc then you are probably using windows and will have to play with your firewall settings (on each pc not the router)

[#3]

This should not be difficult (though there's always something that makes things less easy than they ought to be depending on the OS on each computer).
First, you need to set up file sharing (varies with version of Windows - assuming that's what you're using), for Vista, Windows 7 & presumably Windows 8 you'll need to define the network connection type as 'private network'.
What Operating Systems (and versions) are on each computer?
[Edit] HB got a reply in first