Further research seems to show that XP can also suffer from this (though not as often as 2000).
Apart from slow loading as detailed, XP seemed occasionally to completely lose the plot and not be able to find anything. Flushing the cache occasionally fixed it. Damned irritating. Glad disabling this service has fixed things.
TreeWalk DNS (free) has been suggested as a solution if one needs a DNS cache (it can be a bit slow waiting for your ISP's DNS server) -
http://ntcanuck.com/index.htm