Hi, can anybody please offer some advice?

I am having problems with my laptop crashing during video and occasionally music playback. It is a problem that has happened for about a year now so a system restore is something I an unable to do.
During video playback just before the crash the sound and picture both stutter and fall out of synch with each other. At this time looking at the task manager the cpu usage is at 100%. During playback the cpu usage is between 40% and 70% which still seems high to me. This only occurs with downloaded movies from bittorrent and p2p sites, it does not happen when playing an original DVD. I have run playback through both windows media player and real player, from both my hard drives local and external and also from the cd drive with the same results on cpu usage and ultimately the crash.
I also often have a system crash when performing a virus scan of my hard drive with Norton anti virus which is part of their internet security 2005 pack. I am able to complete about 1 scan in 4 without the crash and it does not detect any viruses on these occasions. During the scan the cpu usage also runs very high at about 60%-80%.

I have an hp pavilion ze5200 laptop with a 2.53 GHz Pentium 4 processor
With 512 mb of ram.
Windows xp home edition.

Sounds to me like the downloads are corrupted in some way.

Perhaps the source of the download also affected your computer Re: Norton's scan.

As to that issue I would recommend running a HijackThis log past our Pros in that section:

Please follow the instructions HERE & then post your log in a new thread in the Spyware, Adware, Viruses and HijackThis Logs section.
(Not in this section please).

Once you get an all clear there we can sort out any remaining issues here.