iMac Slow and Hanging

I'm suddenly having huge problems with my iMac. It's been doing this for 2 days now.

Firstly it won't wake from sleep mode so I need to restart by switching off and on.

Then within about 2-3 minutes it freezes and I get the coloured circle. I'm then unable to do anything until i switch off and on again.

I did get chance to run a quick check of the activity monitor.

This is what I got -

Memory

Physical - 4GB

Used - 3.98 GB (this seems worrying)

Virtual - 4.75GB

CPU

95.28% idle

Disk

In 82/sec - 83,879

Out 0 or 1/ sec - 309

Any advice appreciated