1)Start up the machine log in...
go Start > Run > type "msconfig" > select Diagnostic start up see screen shot click here Apply > OK > Message "Windows needs to restart to complete configuration Yes No" select Yes.
When windows XP Boots it should be in safe mode run a virus scan using Norton, and Spy bot S&D + whatever else you have anti virus and spy ware wise if the result is virus free then i suggest the only choice you have is to take it in for repair check Ur Manufactures warranty or take it to a computer repair shop this fault sounds like it has been caused by hardware! e.g. a lose cable Ur HDD space is not a problem you could try formatting the machine and reinstalling windows if you think it is software related! to boot your PC normally repeat step 1) but select Normal Boot if the PC dose not crash in safe mode i suggest you leave it in it.
Hope this helps if you do get stuck email me on the private response and i will talk you through it using MSN Messenger. Ben Marshall. IT CONSULTANT.