problem with my emachine desktop starting up?

Could anyone help, my emachine desktop has been working fine then I switched it on the other day and rather than go to the windows page a message appeared saying windows could not start properly due to a hardware or software change, it gave me 4 options start normally, go back to last known configurations, start in safe mode or safe mode with networking. I have tried each of these but it just goes in a circle and returns me to the same screen. Does anyone have a solution.
Thanks

James
July 2009