check for spyware? how old is the pc have u installed or changed any hardware prior to this happening?
how much space to u have on ur HDD? how much ram? what graphics card do u have? just give us a full spec of what u have and mayb we can give u a bit more of a clue! also try using a system optimizer! i use advanced system optimizer! or if its a problem u have had recently try using a system restore to go back to a time it was ok mayb?