Have looked at the forum and the overheating option might be the cause- except this happens on cold startup too.
The battery held no charge originally and I have replaced this.
Drivers are all up to date
I used the bootable repair disc and it ran chkdsk and originally corrected some errors. Now it gets as far as a 'Configuring Updates - stage 3 of 3 0% complete. Do not turn off your computer-then restarts repeatedly. Any ideas?