If you've had to reinstall Windows 7, depending on how old your install media is you may run into this problem. Once Windows is installed and ready for updates, you tell it to search and it keeps searching, and searching, and searching... until you finally reboot or shutdown because it's been 5 hours and still nothing. I've ran into this a few times myself, and luckily I think I finally found a fix.
First, download and save Microsoft KB 3138612, which is an update to the Windows Update client. Apparently you now need this update to get the update process to work, but if the process is already broken then you can't get the new client through normal means and need to use the standalone installer. In my case, trying to install from the standalone left it searching forever for installed updates too, so just save this for now. You're going to need this in a minute.
Now, open up a command prompt and run it under the administrator context. The easiest way to do this would be click the Start button, type cmd, then right-click and select Run as Administrator on the Command Prompt or cmd.exe result you should see.
Once Command Prompt opens, type (without the quotes) "net start appidsvc" and press Enter. This should make sure the appid service is running.
Now that the service is running you can go ahead and install KB3138612 from the download you saved earlier. It should only take a couple of minutes to do this. If it gets stuck on searching for installed updates for more than 5 minutes, you may need to find another method. Once the KB is installed, reboot the computer.
Once the PC is on again, repeat opening the command prompt in the administrator context and starting appidsvc. After you've done that, type (without the quotes) "wuauclt.exe /detectnow" and press Enter. That should force the Windows Update client to search for updates and show you results once done.
In my case, it still took a while for it to search for updates because when it came back I had over 200 of them waiting to be installed. However, that was at most an hour. Prior to that I had left the machine searching for updates as long as 48 hrs and it never moved past that status. Once the updated client is installed, and you get past the initial round of updates that will likely be faily large, you should be fine from there to update normally in the future.
