When I installed WAMPServer2.4-x86 on my Windows 7 Pro 64 bit machine WAMP would not start. I was unable to hit the localhost page. Some research on the internet suggested port 80 was being used by another process, blocking WAMP from working correctly. In my case it was a Windows 7 service.
Locate the service using Port 80:
netstat -o -n -a | findstr 0.0:80
The last column will contain the PID of the process using the port. I had a PID 4 which is a HTTP.sys driver for Windows services such as Windows Remote Management of Print Spooler.
TCP 0.0.0.0:80 0.0.0.0:0 LISTENING 4
To disable the service go to the device manager->view->show hidden devices. From the Non-Plug and Play Drivers list choose HTTP->Properties. From here you can stop the service. After stopping it run the netstat command from above again to ensure the PID 4 does not appear again. Restart you WAMP service and it should change from yellow to green!