Sounds like you haven't made any hardware changes?
If you haven't changed or altered your network controller (either physically or thru settings), I'd bet big money the problem is with your AntiVirus or Firewall software.
Both Kapersky and ZoneAlarm have had major issues with this in recent months. Damn bugs. :-)
You need to update/upgrade -- especially if you're running ZoneAlarm.
Warm Regards!
P.S, As always, post back if this doesn't help. There's always Plan B.