Disk Labours Lost ?

My IBM notebook PC has XP-Pro with ServicePack 2. At random times – probably more often with the network connected, but still happens regardless – the hard disk is being constantly accessed even when I am not using the computer at all. This can continue for hours. If I want to switch off and take the thing home while this is happening, Windows might take 15 mins to shut down. The task list (CtrlAltDel) shows nothing unusual ie a long list of processes at 00 CPU usage and SystemIdleProcess taking 99. Am I just supposed to regard this as “normal” XP behaviour. What is it doing ??