iMac running very slow on start up

Hi people, newbie here. I have had my iMac just over a year now I have an iMac7,1 with Intel Core Duo 2GHz processor and 4Gb ram. I still have 210Gb of hard drive space free, but a little while ago it started to become really slow on start up, and I mean really slow. My mac used to be so fast, but now I don't understand what's wrong with it. Once it has finally started and I am in OSX it's really fast again. I am running Snow leopard, and have repaired disk permissions, still the same. Can anybody give me some advice on what may be causing this and maybe a little help on getting my mac back to being fast again.
I hope there is enough info in here.

Sam

Download the free OnyX program and run it to do a cleanup of your Snow Leopard.

After doing that, go to System Preferences, Accounts, highlight your account and then click on "Login Items". See what, if any programs are running on startup. If you have programs running on startup, you might try removing them (if they're not needed) to see if that speeds things up a bit.

Regards.