Sure sounds like you have a hardware issue (one suspect is a fan that is mulfunctioning causing the computer to overheat and shut down) or perhaps a bad video driver.
the pc is fine - its lighroom that is the problem. it used to run properly as i have had it for about 6 months or so - now when i go to start it the whole pc freezes and i have to switch off the power - all other programmes work fine - so launching lightroom is crashing the whole system
Open Lightroom while holding down the ALT key and choose create new catalog.
If that doesn't work than reset your preferences file by deleting it and having lightroom rebuild it.
If that doesn't work reinstall Lightroom and try the new catalog trick.
If that doesn't work then your PC is probably NOT fine and may be a problem with the OS or Hardware.
tried holding the alt key and it's fine - thanks for your help!!!