iPhone 4S does not get detected in Windows
Verify that the iPod service is running by doing the following:

First, close iTunes and disconnect your iPhone from your computer. Then:


Go to Start
Go to Run
Type "services.msc" and hit enter.
Find the iPod Service
Right click on it and click Start or Restart Service
Reconnect your iPhone to your machine. 


If this does not resolve the issue, try going through the steps in this knowledgebase article.