iMac Booting Problems and Running Slow

I own two iMacs, one for myself and another for my family. The day before last they both started to have problems.
First my own iMac booted slowly and then ran just as slowly, now is barely running at all.
The other iMac is displaying the 'press start button' message and won't boot up.
I'm very frustrated since we now have no computers running at all.
If anyone has any advice or information, I would really appreciate it.
Thanks in advance.

Heard of a few probs with a recent Mac update, not sure if that is your problem, but the symptoms are similar.