The error is caused by what the error message says.
It ran out of memory.
@Kagnar - You need more ram, or you need to restart EQ every so often to clear it from the ram. The more you zone, the quicker your memory will fill up as the old zones are kept in the memory with the theory of allowing you to zone back there quicker. And don't have loads of programs running in the background. Close down everything and you will get less 'out of memory' errors.
@Vista users - Vista uses a lot more ram than XP does. If I run my Vista computer with 2gig, I get the out of memory crash a lot. Run it at 4gig and I haven't got one yet
"Why did my computer used to be able to play this game fine, but now it can't?"
Well, two causes. As you use your computer, it slowly gets bogged down with programs and stray 'things' no matter how hard you keep it clean. I found it best to reinstall windows every couple of months to keep it running smooth.
And, as EQ lives on, they add more features, change code, add code, add zones, tweak graphics.....that it will be harder for your computer to process the game. The game evolves and you should 'evolve' your computer every so often with an upgrade to keep it smooth.