iMac running slow..Clean Up

Recently I've noticed my iMac is running very slow. I have the 2.5 ghz intel core i5 with 4 gigs of memory. I checked my storage and I have 244.26 gb free out of 499.25. I also have an 2tb external hard drive. I uploaded all my images to my external and will be deleting the ones off my iMac. I have Lightroom 3 and it too is going so slow as well as my Photo Shop CS5. Is there a way to clean up my iMac hard drive to get it to run faster? It was so much faster before but now it's awful.

First and foremost, you should run Disk Utility to repair permissions on your system and then checkout Onyx which will clean out the rest of your system and perhaps that will make things speedy again..

Disk Utility

What is disk utility? How do I use it?

Disk Utility is an application in your Utilities folder. Select your hard drive in the column on the left then click Repair Permissions and Verify Hard Drive.