iMac running slow

HDD might be crapping out. http://www.apple.com/support/imac-harddrive/

If not: Performance Tips For Mac OS X

If that does not work, provide more information about what OS you use and what exactly is slow and not.

 

Follow the performance tips that simsaladimbamba posted. After you've done those and restarted your Mac, follow every step of the following instructions precisely. Do not skip any steps.

1. Launch Activity Monitor
2. Change "My Processes" at the top to "All Processes"
3. Click on the "% CPU" column heading once or twice, so the arrow points downward (highest values on top).
   (If that column isn't visible, right-click on the column headings and check it, NOT "CPU Time")
4. Click on the System Memory tab at the bottom.
5. Take a screen shot of the entire Activity Monitor window, then scroll down to see the rest of the list, take another screen shot
6. Post your screenshots.

 

Add more RAM, an additional 8 GB will only cost you 50 USD and less (two 4 GB 204-pin DDR3 SO-DIMM RAM modules via Newegg) and then you have 12 GB.

Look at the performance tips I linked to and also at this small guide to see, how Activity Monitor can help you finding the weak link.

Using Activity Monitor to show you CPU and RAM usage​

 

Check out the file system with Disk Utility (perform the 'Verify' option). My iMac did not want to start up last week. At first I though the memory had gone faulty but it turned out the file system had been corrupt. I erased the drive and installed Snow Leopard, then restored my data from the Time Machine. Upgraded the OS to the one which supports apple store and my system has been restored to the original in a very simple and efficient way.

I strongly advise to back up your data before taking any action to your computer. In my case, Time Machine saved 50GB worth of RAW photos of my nephews.

 

What does the System Memory tab have to say?

 

You skipped steps 3 and 4 of the instructions.

 

Try repairing disk permissions within Disk Utility.

You should do this after installing, updating or removing any software.