After 2 and half frustrating days, of trying to figure out what was wrong, getting blue screens, using a very slow dial-up connection, realising my on-line back-up had been closed down, buying an external had-drive to back-up, google searching, registering with new forums to ask technical questions, learning new troubleshooting procedures like event viewer and memory testing, and generally being very, very frustrated and stressed, I've got it sorted.
I went through the Dell troubleshooting, which threatened to require reloading Winows, and the first step was reboot, press F8 and start with last know good configuration. And it worked!
I can't believe it was so simple and that I didn't think of it.
Anyway, Hamluis, thank you for your help and suggestions. I will be sure to come back here if I have any more problems.