I had the same problem in Windows 7 Professional.
svchost.exe ran at start up and it blocked all the system.
I opened the Task Manager (hardly); with the right button, i clicked on the svchost.exe application, which gained more and more of the RAM and the CPU; then I clicked on Open File Location and, surprisingly, I was sent to a folder C:\Users\[current user]\AppData\Roaming\dll, in which there was svchost.exe (otherwise, I had svchost.exe in System32, also). I forcely ended the svchost.exe application, then I deleted all the content of the DLL folder, from the above.
Thus, the system began to work normally.
I think there was a virus or something like this.