computer screen is funny. - computer help forum

hi, i need some help.
okay, so i have this sort of old computer, 2002 XP, that just has games on it. i have another for everything else. but this computer holds all my Sims 2 games, and i know that takes up a lot of room but, anyhow. my computer monitor is a really old one, the old box kind. It's a Compaq one, with the big speakers on the sides. now, when i loaded the Sims 2, the screen was going really weird, it was flickering and it had double of the same thing, or sometimes triple. the mouse pointer was a square with some colours. the sound was fine, but the screen was so bad i couldn't even play if i wanted to put up with it. the colours were kind of wonkey too.
i tried playing another game, and it did the same thing. and the computer screen when it's on the desktop is fine, except a few times it froze on me when i pressed start, and then clicked away, the start menu would still be there.

sorry if i'm absolutely terrible at describing what is going on, but i'm trying my best.
i just want to know if i have to go out and buy a new monitor or if it's a graphic card or whatever...
any help would be greatly appreciated.