This is almost certainly a memory problem.

Go to the Apple menu and select 'About this computer'. At the top, below the system version number you will see the physical RAM you have installed.

Now try opening the programs that you normally work with a (for example Quicktime Player, Limewire and Illustrator) and go back to 'About this computer'. Add up the total memory usage of these programs, plus Mac OS (ie all of the bars that appear in the main window). This total is your memory requirement.

If this number exceeds your installed RAM then you have a problem. Here are some of the steps you can take in this case:

1. Only run programs that won't exceed your physical RAM. If the MacOS uses nearly all of your RAM by itself, then this isn't a reasonable option.

2. Use virtual memory: Go to Control Panels=>Memory , click the 'yes' radio button in Virtual memory (this option is turned on by default) and set the size so that Virtual memory+physical RAM=your total memory requirement.

This option has its limitations. Mac OS 8.6's virtual memory is inefficient and slows things down (you may well already have had it turned on) and if you set the VM size to more than your physical RAM, the performance loss will be excessive.

3. Turn off non-essential extensions, thereby reducing Mac OS's memory usage:

a. Go to Control Panels=>Extensions Manager.
b. Select 'basic Mac OS 8.6' from the selected group menu.
c. Click on Restart

What this does is to activate the minimum OS extensions. This has the side effect that it will disable extensions that you've installed after the MAC OS (such as printer or scanner drivers). If this is a problem or if this action doesn't reduce the OS's memory usage much, you can go back to the extensions manager and select your original extensions selected group, ('My settings' by default). You can also try turning off other extensions you think you might not need (such as Laser Writer if you don't have this printer). This process is more complex, I'm afraid, as you need to know what each extension does. Also, make sure you click on Duplicate Group beforehand, so that you can go back to your previous settings later.

4. The most effective solution unfortunately involves reaching into your pocket and buying more RAM which, if your memory requirements significantly exceed your physical RAM, is the only action that will make a significant difference. For Mac OS 8.6 I wouldn't recommend anything less than 128Mb physical RAM - preferably much more if you want to do image work (Illustrator) , listen to music and use Limewire simultaneously.