Firstly, lets make sure we actually have connectivity to the other machine.
From a command prompt do a ping to the other machine. (ping xx.xx.xx.xx) hopefully you should see a response.
If you don't we need to check a couple of things.
Firstly ensure the subnet mask of the two machines are correct, if they are on the same subnet, then the subnet masks should be the same, if they are on a different subnet, then make sure they both have valid gateways.
If that all checks out, then i suppose we should check to ensure apache is actually running on the second machine, this can be done either through the apache control panel that normally sits in the taskbar by the clock, or via services in administrators tools in control panel.
One thing of note, that has tripped me up in the past, if Skype is running on the same machine as apache, apache wont be able to bind to port 80.
I would also suggest you check your apache configuration to ensure it allows connections from servers other than localhost, if it's a development instance such as XAMPP it will be configured by default to only work locally, though this is easily changed.