You should download the wireless drivers direct from Acer's support site HERE
There are two kinds, either Broadcom or Atheros. Most of the time it's a Broadcom so try that one first.
More than likely a geek has already tried the above solution, if he/she has and you get the same results you can try reseating the wireless card or replacing it outright.
Another thing you could try is booting the computer with a copy of Ubuntu.
Ubuntu is a Live Linux Cd so it won't change anything with the computer but within a few minutes of booting Ubuntu direct from a cd / dvd you'll have a Windows XP like interface that will also have all your drivers pre configured (for the most part).
The purpose of Ubuntu is to help determine if we've got a software issue or hardware issue.
If you want an easy fix you could always just purchase a usb wireless card too.
Let me know if any of this info helps you or if you want more specifics. I don't want to write a novel when a pamphlet will do ;)