It sounds like you are experiencing the 'unclean' startup/shutdown menu telling you the system did not shutdown or startup properly.
Typically the best bet is to turn off the system entirely for a few minutes. Leave it powered down, then start it up. Be very careful NOT TO TOUCH THE KEYBOARD and NOT TO TOUCH THE MOUSE until you need to either enter or type information, like logging in or making menu selections. Make paper notes of the actual errors/crashes that come up, including any funny numbers. This is all important. If it goes by too quickly, I'll use my camera to take a picture.
Try 'Start Normally' a couple times, then if If this 'Unclean' screen keeps appearing, select 'Safe Mode'. This will be one of the options you have. If the normal 'Safe Mode isn't working then you're forced to use 'Safe Mode - Command prompt only'. Depending on your version of windows this may have slightly different wording.
Once you have logged into the system, you will need to review the 'EVENT VIEWER' logs to see 'WHAT' broke or is broken. Start computer management and select the event viewer, or go 'start -> run and then type 'eventvwr' and hit enter, this will load the event viewer either way. Look under 'application' and 'system' for ERRORS, not INFO...just ERRORS. There is a 'view filter' you can use to select just these items from the logs if you wish, view the help for more details.
What kinds of problems cause this behavior?
Always hardware related. Typically adding or removing hardware, video driver or other driver updates, or a failing or failed hard disk.
Seeing what the log shows is the 'real' problem is key to discovering how to fix this. Once you know what is causing it precisely you can seek answers to fix the problem.
Good luck.