auxiliary drive belt slipping off one grove on a citroen c5 petrol?

hi all
i have a 2002 2.00l petrol citroen c5 with 58000 miles on the clock.
i had a new auxiliary drive belt fitted 10 days ago, but it keeps slipping off the crankshaft pully by one grove and i was wondering if anyone has had this problen and how they got it sorted out.
again thanks for all the help so far