Question: I just deployed a website into IIS 7 (about which I am woefully ignorant), and upon trying to build the site, I receive this error. I did a little googleing and I saw an article that said I should put system.web.extensions.dll into the /bin. But, I also saw an article saying not to do that. I tried it anyway, but I just received a different error ('Resource cannot be found').
I am totally clueless as to what else to try

Answer: 
Can you use the "Publish" command in Visual Studio to publish directly to the site? If not, then use that command to publish to a similar site on your machine, then copy it to the customer site. 

You should also look into the [IIS Web Deployment Tool](http://learn.iis.net/page.aspx/346/web-deployment-tool/). It can copy an entire site, including IIS settings and any databases. It will be built into VS2010.
Go to control panel, then programs, turn windows features on or off, scroll down to Microsoft.net framework 3.5.1 expand, make sure both sub options are selected, this might help your issue.