chiltern

if its 'Blue screening' could be - hardware fault (laptops left on sofas,beds tend to over heat and make the RAM duff) - driver problem if you can get into safe mode withou 'blue screening' its probably a driver problem or service - run MSCONFIG and disable all non microsoft services and untick all the startyup programs and restart then if works, then one-by-one re-enable services/startup porgrams and see which one if duff/corrupt. remember - 1 backup user data (documents/email) BEFORE you start any changing of the computer 2 are you insured against unreasonable customers or did they sign a non-disclamer?)

try restoring to a date before you installed the external screen in safe mode... start--all programs – accessories – system tools – system restore and follow the wizard. No data is lost (except any program you installed after the chosen restore date)