Windows 7 tries to create something called a homegroup when it is first connected to a new network. The whole 'homegroup' protocol works only between Windows 7 machines and will make the Win 7 computer invisible to anything but another Win 7 machine configured to be within the same 'homegroup'. You'll need to play with the network setting to get rid of the homegroup and enable the old fashioned sharing model if you want the XP computer to access files on the Win 7 one.
As for Vista, I believe that sharing and security were oomphed a bit and the OS simply doesn't do any kind of sharing by default. I'm not entirely sure if that's the issue for you but it sounds rather plausible.
P.S. To get rid of the homegroup you do the following (preferably while the Win 7 computer is connected to your home network):
- Click on the connection's icon in the notification area
- Select Open Network and Sharing Center
- Select Homegroup and Sharing Options
- Select Change Advanced Sharing Options
- Assuming your home network has been marked as a 'safe' (home or work network) you want to change the options under 'Home or Work'
- Enable network discovery and the anything else you might need (file and printer sharing, public folder sharing, media streaming, etc.). I'd also recommend enabling 128 bit encryption for extra security but if you're still having issue turn it down to 40 bit encryption.
- If you want to do file sharing also choose to use password protected sharing and select the 'Use user accounts and passwords to connect to other computers' option as well.
