It sounds like the issue most of you are having is well covered by the fixes mentioned in the posts above.
I'll mention an oddball but potentially-effective thing to try if you experience problems related to wifi on any of your devices or computers- power cycle your wifi router (turn it off for 15 seconds and then turn it back on).
Here's how I discovered it. We seemed to have lost internet to all devices so I called the helpline of our ISP. They suggested to power cycle our router... I thought it was another silly waste of time but tried it and it worked to restore our internet. I'm now finding it cures a lot of other oddball intermittent things related to our connection. For example sometimes I can't connect from my phone (ES file explorer) to Windows 7 shared network folder on my pc... it's cured by power cycling router. Sometimes I get periods of frequent disconnect (page did not load)... cured by cycling router.
It may not apply to everyone but for us it is an easy fix to most wifi problems (power cycle the router). We have a Belkin N150