we apologize for the inconvenience, but windows did not start successfully on dell optiplex gx260?

Ok so my parents have a dell optiplex gx260 that they gave me at school. But now the problem is that my dad was messing with it and when I start it up it shows the welcome windows start up page but then, it shows me a message saying we apologize for the inconvenience, but windows did not start successfully etc. And it gives me four options to start in safe mode, safe mode with network, safe mode with command prompt, and start windows normally. I clicked start windows normally and all it does it takes me again to the welcome windows home page and gives me the same message again and I have already try ed all the options above and it would do the same all over again. The only other options it also gives me its to click the F2 and F12 keys but I don't know what to click or fix. Please help me ASAP thanks a lot!!