Freezing Problems?

I am using an iMac 20", 2 GHz Intel Core 2 Duo, 1GB memory, V 10.5.5. The problem occurs with any program running. What happens is the keyboard (wireless) become unresponsive and the mouse (wireless) pointer is able to move but not click on anything. No beach ball on the screen. This forces me to shutdown from the power button. This has happen 4 or 5 times once during the screen saver. Any ideas of what I could do to fix or prevent this from happening again?

Thanks for all the help offered!

Running other software such as SteerMouse or USBOverdrive by chance?

What does Apple System Profiler say about Bluetooth, especially firmware?

Running other software such as SteerMouse or USBOverdrive by chance?
No

Firmware Version: 3.1965 (3.1965)