Why is laptop running so slow when turned on. can not be used.

Sounds like your hard drive and/or your hard drive cable are failing. The best thing here is to take the drive out and use an external case or SATA to USB adapter to then hook your drive to a second Mac so you can first copy off anything important and then run Disk Utilities. If you can I would recommend getting a defrag program to defragment your drive or you can just reformat the drive and re-install your OS, apps and your saved data back. I would just to be safe replace the hard drive SATA cable while you have it open and give it a good dust out as I'm sure its dusty!

Here's the IFIXIT guide to replace the cable: MacBook Pro 15" Unibody 2.53 GHz Mid 2009 Hard Drive/IR Sensor Cable Replacement.