This issue is caused by the pipeline mode in your Application Pool setting that your web site is set to.
Short
Simple way (bad practice) Add the following to your web.config. See http://www.iis.net/ConfigReference/system.webServer/validation
<system.webServer>
<validation validateIntegratedModeConfiguration="false" />
</system.webServer>
Long If possible, your best bet is to change your application to support the integrated pipelines. There are a number of changes between IIS6 and IIS7.x that will cause this error. You can find details about these changes here http://learn.iis.net/page.aspx/381/aspnet-20-breaking-changes-on-iis-70/.
If you're unable to do that, you'll need to change the App pool which may be more difficult to do depending on your availability to the web server.
Check http://technet.microsoft.com/en-us/library/cc731755(WS.10).aspx for details on changing the App Pool
If you need to create an App Pool with Classic pipelines, take a look at http://technet.microsoft.com/en-us/library/cc731784(WS.10).aspx
If you don't have access to the server to make this change, you'll need to do this through your hosting server and contact them for help.
Feel free to ask questions.