I used to have a Dell laptop and I used to have intermittent trouble connecting wirelessly via a D-Link access point to the internet.

For ages I blamed the wireless card, or the access point, until one day I figured it out it was actually Windows XP being stupid:

Next time you are having trouble connecting, do this:
Start
RUn
cmd
ipconfig /flushdns

And try connecting again.

The problem with XP is not that it won’t connect (try connecting to an IP address instead of a Name on the internet somewhere and it will be fine), but that it decides it can’t translate names to IP addresses sometimes.
[technically: it caches negative DNS responses].

If you create a .bat file on your desktop with the ipconfig /flushdns command in it, you can quickly run it anytime you like. (run notepad, put the line in, then save it as .bat instead of .txt).

It is also possible to edit your registry to tell XP to NOT cache negative DNS responses if you want to get really on top of it.