Right or control click the MacintoshHD icon on your Desktop then click Get Info.
Look at Capacity and Available. Less than 10% free space on the startup disk can slow the drive down.
If that's not the issue, check System Preferences > Accounts > Login Items
If there are any apps listed there, delete and restart to see if it makes a difference.
And check the HD > Library > StartupItems folder. Same thing.
Indexing Spolight can help to speed things up a bit > Spotlight: How to re-index folders or volumes
Which model iMac are you using? http://support.apple.com/kb/HT1758 How to identify iMac models