Does turning off the firewall in the router help?

Also, check the firewall settings on both computers. IIRC in Windows 7 ping (ICMP) is blocked by default and has to be enabled for this to work.

If you can, use the home network feature in Windows 7 and put both computers on that network.

are u connecting those desktop on a wireless network or with cable ??

you might need to go to the advanced settings of ur wireless router and uncheck AP Isolation to enable accessing/sharing with other computers on the wireless network , and i guess win7 will configure itself or ask u about sharing , u might need to enable sharing in windows xp by urself , if you dont know you can always google there are tons of tutorials on the subject .

Hello- Same problem here.

When I use a different router with the same machines, everything works fine. I have tried checking and unchecking AP isolation and it still does not work. All the machines can ping the router, but cannot see eachother. File sharing is setup correctly because when I connect to a different router, it works fine.

To be clear, keeping everything the same, when I use my TL-WR700N I am unable to connect to the other computers on the network.

Any advice? Thanks!

As both the computers are on different network, you need to use 3rd part remote support tools such as logmeinrescue, gosupportnow, GoToMyPC etc. in order to remotely access computers over internet. Another option is deploying on premise remote support appliance such as RHUB or Bomgar appliances.