Blinking screen

Friends, Pls help me.

i have a dell inspiron n4050 laptop. i was not using it for 2 years. now the issue begin with booting. after consulting i changed hard disk and put new toshiba 500 gb hd. after few weeks, when i switch on, it boot with some sound for 3-4 seconds. After it started up, sometime screen flickers and even curser would blink. i am unable to use Word for editing. what would be the reason. will it be keyboard problem? how can i solve it? i fed up with wasting money for servicing... tell me a solution