For the last few days there have been horizontal lines at the top of my desktop. I can't seem to get rid of them, they also appear when I play a game or watch a movie on my computer. They crop up when I change the width of the screen.

Any idea how I can get rid of them?

ps. I tried to take a screenshot of it, but they don't appear on screenshots.

What's your videocard, and also your monitor is it LCD or cathode?
What was the last thing you remember installing when that problem arised, maybe you should try changing drivers of your videocard...

I've got a G4Ti4400 which, up till now, hasn't given me any problems.
I've already tried updating drivers and I can't even remember when the problem began showing up (I think I've had it for some time but the lines would disappear by themselves, now they just stay there... mocking me)

If the lines are not on the screenshot, then it seems that it is Your monitor that causes troubles. Check, if You can, whether the problem remains when You connect the monitor to another PC. If yes, then it must be something in the hardware - You'll need to get it fixed.
Perhaps it is old; maybe it does not support certain resolutions?

horizontal lines? yes, it could quite possibly be that your resolution settings need tweaking

It could also very well be the first forward sign that your monitor is about to blow. Sometimes it take a lot of time, sometime not.

Try to adjust the height and other settings to see if it stop the problem. If not then do like above and try it on another computer. If you can't fix the problem get a new monitor asap.

Oh woohoo, my brother tells me to buy a new graphics card and you're telling me to buy a new monitor... this is becoming expensive to say the least :D

who says you need a graphics card? this pc dosen't have one and I can run call of duty on it ^^
how on earth that works out I REALLY dont know

Do screenshots take their input from the gfz card or from some point before that (if it's the latter it makes it harder to diagnose...). If they're always at the top of the screen independently of the resolution that suggests to me (though I know nothing about hardware) it's more likely to be the monitor. Is it a CRT monitor? They do things like that when they're on the way out.

I believe I do have a CRT-screen (compaq FS940)
However as I said, when I make the screen smaller (top and bottom closer together) the lines at the top come closer together (less distance between them) until they become one thick line.