Is this a wired or wireless connection issue?
Either one have you tried disabling then enabling the connection? How to Disable a Wired Connection From a Desktop Computer | eHow.com
If that didn't work, please provide us an ipconfig /all output for review.
Click on Start => in run or search box type cmd, choose run as Admin and press enter. Open up a command prompt then Copy and Paste these ipconfig /all >c:\ipconfig.txt . Please attach the .txt file to be found in your Local Disk 'C' on your next post.