UPDATE TO WORKAROUND
I started my XP image after not using it for over a week; this time while svchost.exe was using 99% Windows CPU, I was able to find the offending service in time: I ran net stop wuauserv , the Automatic Updates service stopped, and CPU usage dropped to normal. (I haven't applied the Parallels update yet, so all this might no longer be necessary, but I'm documenting it in case it doesn't, or in case users have older versions.) While typing al this, the service started itself and pegged the CPU again. I stopped it, changed Control Panel, Automatic Updates from download to notify, no change; set to Turn off Automatic Updates, and it stayed dead. (The service shows as not running in Services; if a reboot starts it and pegs the CPU after the Parallels update, I'll disable it from there.)
Update: I applied the current Parallels update; the only change to the above issue was that in Mac OS' Activity Monitor, it no longer says Not Responding. Also, in Windows XP, if Control Panel, Automatic Updates is turned back on from an off state, the Automatic Updates service's startup setting is changed from Manual or Disabled to Automatic, and it starts (and within about 10 seconds, svchost.exe goes to 99%). Setting Automatic Updates to Turn Off, and setting the Automatic Updates service to Manual or Disabled, should stay off after a logoff or reboot cycle; I didn't bother testing it.
Last edited: Nov 19, 2013