HELP!

Bought an xbox off a mate a few weeks back he's had it for year or so. Everything ok until yesterday....

Everytime I start to play a game the screen goes all messed up, yet when i pause the game or go to the xbox menu the screen is perfect. Likewise as the xbox boots up then the game comes on on i notice lines going through into seqence then when some games I have start its visably unplayable but the xbox sub menus are fine????

I dont know if the warranty is still valid after a year, any advice would be welcome.