I've seen this when the WiFi is WPA or less security. Until last month I only concerned myself if the security was WEP or less but there are new tools to hijack and interfere with WPA or less protected networks and before you ask, said software will only cause issues for a single machine at a time.
To rule out if there is a hardware issue, try the laptop at a free hotspot.
Bob
In our house we have 4 MACs that all have been accessing the internet (via Safari) very well over a wireless router for about 4 years. No problems with any of them.
Suddenly, one of the computers (an IMac11,2 running OS X, version 10.6.
is almost completely unable to open web pages, download, etc. Nothing was changed on it. Am running Safari 5.1.7. The signal strength is at the maximum. The internet diagnostic under System Preferences says all is fine. A laptop in the same room has no problems.
I've done a virus scan (nothing). It made no difference when I used Firefox instead. I'm at a loss. Any suggestions?
Thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic