Win2000 machines running slow

Within our office we have a couple of High spec notebooks (1GHz, 256M ram) runninng Win2000 we have found that these run very sluggishly on our network (Server is Win NT 4.0 SP6) almost taking twice as long to open files than the Win98 SE machines. Can anybody explain why this is happening, and are there any ways that we can resolve this problem.