System Process Taking Up Too Much CPU

This is not my pc, but my parents'. It is a Compaq Presario. It has a P4 3.06Ghz cpu, 1GB of ram, and runs XP Home SP2. I remember when my parents first got the pc it was very fast and stable, now however they complain of two problems.Firstly it is really slow, especially if your mouse pointer hovers over the taskbar or if you attempt to do most things with Explorer, like right-clicking. Sometimes this can lock the pc up for minutes at a time. The second problem is that it doesn't shut down no matter how long you leave it. The best it manages to do is remove power from the keyboard and mouse, and then only occasionally.I Have investigated the problems with the speed of the pc, and the problem seems to be to do with the system process. Task manager reports this process as using anywhere between 40% and 99% of the CPU all the time.My actions thus far have been to do a complete sweep with anti-spamware and ant-virus, and they didn't find anything. Next I ran process explorer, which showed that the system was using about 50% on deferred procedure calls, 20% on hardware interrupts, and 30% on the System process itself. Upon looking at the threads running in the System process, I discovered that the thread ACPI.sys+0x10b10using the entire 30%.

I must admit I don't really understand what ACPI is, but an internet search revealed it to be associated with pcs being unable to propperly shut down. For this reason I am now assuming that the two problems (unable to sut down and running very slow) are related. This is just an uneducated guess however.So can anybody suggest what the problem might be? Or how I might solve it?If you need any information that I have forgotten, I will gladly provide it.