cleaning out of ALL dust in the case - 10 seconds with a can of compressed air..come on that should come free with service.
cleaning the registry
ad/spyware removal
virus removal
defrag - same thing as others
check disk - same thing as others
crap clean out (temp files, cookies) shouild come free with service like 30 seconds.
I say 20 dollars an hour. Thats good pay, cheap for them so they will use you again later.