are you using a laptop or a base computer? does this happen during boot?(startup) or when u play games or open a specific application, and it should provide you with a error number that you could google.
if its boot:
most probably a driver problem, try a safe boot then if that does nothing uve got a virus... new harddrive or worse(better imo) a new computer
if its games:
theres something wrong with ur graphics card. remove it and boot again. if u dont know what ur graphics card looks like it might be written on the case.. google image and look inside..
bit more info net time plz