Is there anyway of finding out which app is using which instance of svchost.exe in Vista?

I have (including all users/local/system/network) 19 instances running, and one either consumes lots of memory or lots of CPU.

search for process explorer, it should show you what you need to know. You want the sysinternals one and not anything else.

Thanks, I'll have a look tomorrow.

Spybot will show you what processes are running

For the old-fashioned option use "tasklist /svc" in the command prompt, or alternatively right click on the "svchost.exe" entry in Task Manager and select "Go To Service(s)" and it'll highlight the ones corresponding to that instance.