Also, in addition to Furry's response, dlinkap.local. may not work with XP for some reason. Try "http://dlinkap". And if i remember right in some cases make sure you add the period at the end of local.
To ping, use a Windows computer that is wirelessly connected to the extender.
Click on Start (or tap the windows key on your keyboard). In the "search" or "run" box, type cmd and press enter.
A window will open with a black background and blinking cursor. Type ping 192.168.0.1 and press enter. This is assumed your router has this IP address. If you get 4 "reply for...." messages, it is successful. If you get something else like timed out, general hardware failure, etc, then it failed. If you do not know your router's IP address, type ipconfig and press enter. The IP address next to Default Gateway is the IP address of your router. Ping that IP.
If it passed, try ping 4.2.2.2 and press enter. If it failed, computer cannot get to the Internet at all.
If passed, type ping yahoo.com and press enter. If this fails, something with the DNS settings on your computer. If it passes, You are successfully connected to the Internet and there may be a browser issue.