VS - unable to start debugging on a web server. A debugger is already attached.

I'm using VS 2010 and Windows 7 with IIS 7.5. I'm trying to debug a site on localhost. This used to work. Now, I get the "debugger is already attached" message when using F5, and when I try to manually attach, the wp3wp.exe is disabled.

My process list shows 3-5 instances of DbgHost.exe. If I shut them down, one of them takes the wp3wp.exe process down with it, so clearly, that's the debugger process that was attached to my worker process and preventing VS from being able to attach. The question is, what is causing DbgHost.exe to attach, and how do I get it to stop? When I restart the worker process by simply browsing to the site, DbgHost.exe once again automatically attaches, so I'm back to square one.

Any advice? 

Thanks,
Sean