"Dazed" says you can only take one action on your turn aside from free actions. If you get dazed after taking an action, you cannot take any further actions that turn, because you've already taken one action on your turn.
Likewise, if you start your turn dazed, and use your one action to remove the Dazed condition (i.e. by triggering an extra saving throw), you can then take further actions, because you're not under any restriction as to how many actions you can take.
I'm not so sure its as clear cut as you think. Once again, slow states the following:
Your speed becomes 2. This speed applies to all your movement modes, but it does not apply to teleportation or to a pull, a push, or a slide. You can’t increase your speed above 2, and your speed doesn’t increase if it was lower than 2. If you’re slowed while moving, stop
if you have already moved 2 or more squares.
The bolded part is my emphasis. The slow condition is explicit that it looks back in time at your movement - and if you have already moved two squares you stop movement.
Dazed, to use your example, says the following:
DAZED
✦ You grant combat advantage.
✦ You can take either a standard action, a move action,
or a minor action on your turn (you can also take
free actions). You can’t take immediate actions or
opportunity actions.
✦ You can’t flank an enemy.
Nowhere does it look at what you've done prior to the condition going into effect. Dazed does not explicitly look at what happend prior to the condition going into effect. Furthermore, if it did, it would make for some strange rulings if someone had already performed more than 1 action. Would you roll back the action? Would you just ignore the other action? Either way, it creates a bit of a bizarre situation. The only logical option is to apply the effect from that point on.