On the desktop where you should be HOSTING the file you'll need to share the "Company Files" folder. Once that's done, go to your laptop and create a network drive pointing to that particular folder. After that you can go into QB and tell it where to pull the file from.
Depending on your license from QB determines if you can use the file at the same time on both machines.
The "Company Files" folder will be under the "Public" (Win7) Users folder or the "All Users" (XP) folder, then documents>intuit>quickbooks>company files.
For the record though, Quickbooks doesn't recommend running the program over a network. But I havent had any issues really with that part so you should be ok.