It does sound like a DHCP issue.
Is the Windows 7 device a laptop or a PC? Certain laptops may not like AES. Try setting a static IP address on the Windows 7 machine to see if it fixes the problem. If not, try changing the wireless security to TKIP (Take a backup of your routers config just in case).
List and Name all Anti-Virus, Security or Firewall Software installed.
Install and run
Xirrus Wi-Fi Inspector.
Double click the Xirrus Icon on your Desktop, Tap the Printscreen key on your keyboard, open up Paint then choose Edit from the Menu and select Paste, then SAVE it. Upload and attached it here, click on Go Advanced and locate Manage Attachments Tab and attached the file.
Post an
ipconfig /all of the computer in question for review.
Note: For Vista and Windows 7, you have to open up the command prompt in an elevated command:
Click on the Start Menu search box area type
cmd, right click on cmd see at the top, and click on Run as Administrator.
Click on Start => in run or search box type
cmd press enter (run as Admin for Vista and 7). From the black screen, type the word
ipconfig /all and press enter. Right click on the black screen or Command prompt then choose Select All and Paste the results onto your Post.
You may also attach the
ipconfig /all output if the computer is unable to connect to the Internet then use an external USB flash drive to transfer the
.txt file.
Click on Start => in run or search box type
cmd, press enter. From the command prompt (black screen), 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.
Dave