Old Dell XPS r400 running XP .Had cause to do a repair inastllation,some thing has gone wrong.When I switch on at first got the Dell logo then Windows opening within seconds I got a black screen.O the black screen is the mouse pointer but the light on the keyboard goes out.Tried to start with the start disks,got as far as the repair console but no further, any ideas Regards Donald

Old? Yeah, 10 years old is old. How old is the battery? If you don't know, replace it. Is everything, including the current drive properly identified in the BIOS Setup Menu?

Why did you have to do a repair install?

I would remove any extra drives and USB devices before trying to reinstall. Is the RAM good? If you don't know, use 1 stick at a time. You can also test your RAM using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Using the floppy method is generally easier and yet another reason I still include floppy drives in my own PC builds. However, the CD method is just as effective at detecting RAM problems. Allow the diagnostics to run for several passes or even overnight. You should have no reported errors.

Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
or
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.