Wow I had to just post this to thank Jumpa for that solution. My friend's Acer Aspire One D257 just would not connect to the internet and this solution worked when nothing else would. If you don't mind Jumpa, I'll clarify the fix for Windows 7 users.
1) Right-click on the wireless network icon next to time/date
2) Select 'Open Network and Sharing Centre'
3) Click 'Change Adapter Settings' on the left
4) Right-click 'Wireless Network Connection' and go to properties
5) Uncheck all the boxes except for 'Internet Protocol Version 4 (TCP/IPv4)
6) Click ok
It wouldn't hurt to update the wireless driver as well though.