Dialup modem, can be pinged but not ping

I am having a problem with my mothers computer, which is one of the problems since she lives 300 kilometers from me...

When I was visiting her she said "Internet isn't workin!". So, here we go again...

The dial-up process works fine, I get an IP from the ISP but I can't ping it. I tried disabling the antivirus software (Avast), uninstalling Avast and disabling XP's firewall (restarting the computer between every try).

After a while I gave up and called the ISP to see if the problem was with them. After all the standard procedures ("Have you turned the screeen on?" and so on...) I got her to try to ping my machine, and that works fine. However I can't ping anything.

The computer is running XP and probably SP2 (a bit unsure).

Do you have any suggestions? If you do, please post all of them since I will need to try to guide my mother in the process or go there to test them.