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[Wula]

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[Benjamin]

That... sounds like a graphic card issue. Sedy had something like that happen with her own recently.
If your board has integrated video, try plugging your monitor into that jack instead, and see how it goes. You might have to tug out your graphic card before you can get any results from the other port though.
 
Benjamin

[Okami]

There are a few possibilities:

1. You need to re-seat your VGA card.
2. Your VGA card is over-heating.
3. Your VGA card drivers need to be updated.
4. Your VGA card is dying/dead.

My guess is either 1 or 2 considering you get evenly spaced vertical lines. Worst case is 4. 3 is unlikely (since it was working fine before) but still possible.

[Kesarra]

Reseat your card AND power down your monitor and check its connections. Make sure when you mess with the vid card that you screw it down firmly and the monitor cable doesn't jiggle the card loose when you reattach it.

[Mobile Suit Bunny]

You overclocked any bits of your computer lately?

[Kesarra]

Do you have a spare card to try? There's always the chance the motherboard is at fault.

[Alex]

Does it act like that while the computer is booting? Or only when you get into Windows?

[AshleyAshes]

Does the fan and such on the card still operate?  If it has gotten suck from dust or just wear and age, it could stop running and not be cooling the card sufficently.  Resulting in overheating.

[Aaeden]

Upgrade and take it out later when you're building a new one, just make sure the graphics card is of a current standard, PCI-E if your current computer supports it, unless you're a hardcore AGP fan. You're not likely to find a newer motherboard that kicks but AND supports the old AGP format, if I'm correct.