Netware Client for Windows – where does it come from?

I support many Win XP PCs for a charity and have now come across two PCs on different sites on separate occasions that have had Netware Client installed ‘somehow’. This blocks the friendly Welcome screen. Googling the web provides many pointers for it’s removal but I can’t find anything that says how it got there in the first place. Is it a sort of downloaded trojan that allows entry via a Client software vulnerability? Does anyone know likely sites that may have downloaded it. I’d like to avoid it happening again but cerainly AVG Internet Security doesn’t spot it being installed.