Thanks for all the advice. I’ve checked, and have now got virtually nothing in the Start-up menu. I’ve turned off Allow Indexing Service. Cleared out Temporary Internet Files. Unfortunately, not a lot of difference so far.
It still gives a problem from time to time with 100% CPU usage for no reason, and a gradual build-up of PF usage to beyond the 1Gb mark, when I’ve left the computer on for a couple of days. When I turn it on, and back on again, everything’s back to low level usage.