If you're one of those folks who pines for the days of level 60 and Molten Core, but you've already installed Burning Crusade and made your way to 70, Slorkuz (who?) officially tells us you're out of luck. Once an account has been upgraded to Burning Crusade, there is no going back -- even if you uninstall everything and just put vanilla WoW on your PC, trying to sign in with your BC upgraded account will open up Outland and the latest content yet again.
If you really want to see the world the way it was before the Dark Portal reopened, you'll have to create a new account, and not upgrade it to Burning Crusade. But even then, you won't really go back in time -- you'll still see gems on the AH, and Blood Elves and Draenei wandering around. It's one more reason to keep asking Blizzard for classic servers, apparently -- as of right now, there's no way to really go back to the way things were.
But why would you want to anyway, right? Gold abounds, epics are easy to get, and most of the PvP problems are fixed. Nostalgia may be telling you that you want to stumble around with 39 other people in Motlen Core hoping for a Tier 1 drop every two weeks, but for most people, things are much better nowadays.