could anyone help. i am not too bad on computers but not an expert and i am new to this type of forum so please bare with this thick head . here is my computer spec first

i have a 200 gig hard drive with a pentium 4500 3.4 gig prosseser a radeon x600 series
i have an external 160gig hard drive
and 2 gig ram of memory
the rest is not important. as u know
what is happening is my computer crashes in fact everything freezes. not even a mouse pointer and the only way i can turn it off is from the mains supply. i have run an antivirus an anti malware and a anti spyware... nothing it still does the same. the problem is intermitent and dosent happen all the time (excuse the spelling a but drunk . sorry.
i have done a system restore i have done a reinstal of the operating system (which is xp pro) and i have formated the mbr. still the same problem. the concusion i came to is that some of the clusters are damaged on the hard drive and might need a new one anyone help please

have you run memtest? sounds like you could have some bad memory on your hands.... you could also check the temp of your cpu, it might be getting too hot.

Here's the link for Memtest, that mdxchaos mentioned.
Also, go to the harddrive manufacturers website, and download their disk utility, to check the drive out.

Also, check your Event Viewer for error messages. Here's a guide on it, but if focuses on BSOD's - so you'll have to look for more errors than just the one's mentioned there. http://www.bleepingcomputer.com/forums/topic40108.html

thanx for ur tip i looked at the event viewer and there are loads of errors in the service control manager what ever that means is this the problem do u think?

Double click on the errors and then copy and paste the text from the window to here. Start with the last error and work your way back through them. You may notice a pattern to them - so we won't need them all - but we will need enough information to see what's happening.