Look for "slow" in the search field and you will find it gets asked rather regularly.
Backup for starters.
Get a FW drive and a new internal drive.
Install a fresh copy of OS X to an emergency boot partition on the FW drive.
Backup to the 2nd partition of your FW drive.
Repair the boot drive with Disk Utility and Disk Warrior.
Keep your boot drive in good shape by running Disk Warrior weekly.
Backup using SuperDuper weekly to one drive, daily to another pair of drives.
Use Tiger Cache Cleaner and do what it calls "deep clean cache folders" AFTER you know it is running fine.
Every couple months, do your backup and then erase your boot drive and restore.
Always backup before updating.
Keep your boot drive to 50% free space.
use a couple drives if possible for your data.
After a couple years, retire your drive and use it for backup.
Maxtor MaxLine Pro 500GB $139 would be one choice.
Samsung and WD RE2 or SE16 500GB $140-165.
Maxtor MaxLine III 300GB $77 is back at OWC great little drive
http://www.macsales.com
Do a SAFE BOOT with shift key held down.
But do setup an emergency boot drive to repair your drive. And get your backups in order.