Two computers on a network can't see each other(sometimes)

I have two computers, a desktop using windows 7 and a laptop using windows xp. I am hooked up to a Linksys router and I can access the internet on both computers at the same time and connect both computers to the same game using LAN. The only problem I'm having is getting the 2 computers to share files with each other consistently. That is, I can get them to see each other about 50% of the time.

On the windows 7 one, if I go to the network and sharing center, I see a connection of:

desktop (windows 7) computer ----->network 3------->internet.

when I click on see full map it shows the connections of

local area connection 2- network 3 and below that shows

desktop (windows 7) -----gateway------internet.

at the bottom it says "the following discovered devices cannot be placed in the map" and that device is my laptop.

If I click on the laptop icon, it will let me share files. The problem is sometimes it doesn't even show up at all hence, i can't share files. Anyone know why it only shows up some of the time and how i can get it to show up all the time?