Q: My (iMac 3.2 Ghz, i3) is running very slow.

What OS are you running? if its 10.7+

go to the apple symbol and click system preferences

then click security. On this main page you can see "Allow apps to be download from" and select the option anywhere. You will now be able to download the app your colleague said to download.

Get into safe mode:

turn on the computer and press and hold the SHIFT key on the keyboard. Hold it down until you see a bar loading underneath the apple symbol. Then release once you see the bar loading.

In safe mode open finder, Go to the top tool bar where the apple symbol is and click the word GO

Now press and hold option on the keyboard, library should appear in this drop down menu. Click LIBRARY.

In library look for the folder caches, move everything inside of this folder to your trash.
open FINDER again. Go back to the drop down menu GO. Open up UTILITIES.

now look for DISK UTILITY. open it. On the left hand side select macintosh HD. on the right hand side click verify disk.

If no errors occur, go to the apple symbol and click restart to start the computer up normally. If error's due occur you have to get into recovery mode to try and repair them. Here is how to get into recovery mode: http://support.apple.com/kb/HT4718

all the best.

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.