, Welcome to Bleeping Computer
Are you using IE 6 or IE 7?
First, try and clear the DNS cache manually.
Go to Start/ Run, type type in; cmd
and at the black command screen type ipconfig /flushdns
Start / Run / type in; regsvr32 urlmon.dll
Test if you can browse the web.
There are many possible reasons why this is happening, and the contributing factors vary for each computer.
Go to the Control Panel then Network Connections.
Click once on your active Connection.
From the menu on the left, select Repair This Connection.
Try to connect to a site.
Is there any Proxy set or used?
Check to see if set to automatic:
In Internet Explorer,
Go to Tools/ Internet Options/ click on the Connections tab, then click on LAN Settings.
Select Automatically Detect Settings.
Click Okay twice.
If still no joy, clear old files and settings
that may be preventing you from connecting to the Internet.
Select Internet Options.
Click the General tab.
Click the Delete Files button.
When the box opens, put a check in Delete All Offline Content.
Click Delete Cookies. When you are prompted to confirm this selection, click Okay.
Click View Objects.
Click View, and then click Details.
If Damaged appears in the Status column for any program file, remove that program file.
Close the Downloaded Program Files dialog box.
In the Settings dialog box, click OK .
Under History, click Clear History, and then click Yes when you are prompted to confirm the selection.
In the Internet Options dialog box, click OK.