Sound but no picture - sometimes

W 2000, ever since I,ve had this computer this has been a problem but never bothered before as I dont use it much.
My best example is a film clip sent to me by email, some days its fine, next day it will only play sound, but on the 3rd day its OK again.Once Media player works it will work all daywith anything until switched off. If it wont play then it wont play all day. This happens with all sorts of files.
When I start the video the WMP screen is blue, when the sound starts the screen goes black. What can it be? My user name is Dumbo 'cos I’m not into computers much so be gentle with me!!