If its an update you'll probably be right Clive, but I didn't think it did that.
Indeed the task manager should tell you nicely what task is using up all of his HD.
How much RAM has he got, is any of the tasks using lots of RAM. If its losing lots of RAM then the machine will using the hard drive as SWAP space and will be constantly reading and writing to the disk. This is much slower than using RAM and so not only will the disk io be high it will also cause a noticeable performance loss (well much less so in the case of a RAIDED or solid state system).