My Windows 7 Pro 64-bit, on occasion will slow to a crawl. Apps take minutes to come up and then can't be stopped as they aren't responding. In this state the machine won't close down properly either. This happens intermittently - it can run for days without a problem.
According to Malwarebytes and Avast! there is no malware on the machine.
In this condition, both my cleaning programmes lock up at the 'deleting temporary files' bit, but after a reboot they may well run and delete the temporary files after which the machine behaves normally - until next time. I noticed in some forum or other that deleting temporary files can clear a slow machine. I thought that this may be the problem, but now I'm not so sure.