My four year old iMac is running much slower than when it was new. Any suggestions on cleaning out the cob webs?

Question

My four year old iMac is running much slower than when it was new.  Does anyone have any suggestions on what I can do to "clean it up" and get it running like it used to?

Answer 1

What year, screen size, CPU speed and amount of RAM installed?
To find out info about your system,
Click on the Apple symbol in the upper left of the OS X main menu bar. A drop down menu appears.
Click About this Mac. A smaller popup window appears. This gives you basic info like what version of OS X your iMac is running, the speed of your iMac's CPU and how much RAM is installed.
Click on the button that says More Info. A larger window appears giving you a complete overview of your iMac's hardware specs.
Highlight all of this info and copy/paste all of this into another reply to this post, editing out your iMac's serial number before actually posting the reply.
This will tell us everything about your iMac so we may begin to help with your iMac issues.
How full is your Mac's hard drive?
Locate your iMac's hard drive icon on the OS X desktop. Click the icon once, then use the keyboard key combination Command-I. This will give you additonal info about your iMac's internal hard drive.  
Post this info in your reply here, also.
Here are some general tips to keep your Mac's hard drive trim and slim as possible
You should never, EVER let a conputer hard drive get completely full, EVER!
With Macs and OS X, you shouldn't let the hard drive get below 15 GBs or less of free data space.
If it does, it's time for some hard drive housecleaning.
Follow some of my tips for cleaning out, deleting and archiving data from your Mac's internal hard drive.
Have you emptied your Mac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you store images in other locations other than iPhoto, then you will have to weed through these to determine what to archive and what to delete.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Delete any old or no longer needed emails and/or archive to disc, flash drives or external hard drive, older emails you want to save.
Look through your other Mailboxes and other Mail categories to see If there is other mail you can archive and/or delete.
STAY AWAY FROM DELETING ANY FILES FROM OS X SYSTEM FOLDER!
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive to disc, flash drives, ext. hard drives or delete any old documents you no longer use or immediately need.
Look in your Applications folder, if you have applications you haven't used in a long time, if the app doesn't have a dedicated uninstaller, then you can simply drag it into the OS X Trash icon. IF the application has an uninstaller app, then use it to completely delete the app from your Mac.
To find other large files, download an app called Omni Disk Sweeper.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its initial automatic tests, then go to the cleaning and maintenance tabs and run the maintenance tabs that let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.
If you have any other large folders of personal data or projects, these should be archived or moved, also, to the optical discs, flash drives or external hard drive and then either archived to disc and/or deleted off your internal hard drive.
Good Luck!