Hi and welcome to the forum. :flower:
Your Windows installation has become corrupt and needs to be repaired.
This can be caused by viruses, trojans or a number of other different reasons such as improper shut downs or failed updates/installations etc.
You can repair it if you have an installation disc but since you haven't then you'll probably have to restore the machine to factory status using whatever method your PC manufacturer provided.
If it's as a result of a virus or other malware then you're probably better doing this anyway.
Don't forget to back up any data you need first though.