System has become unresponsive, constantly hanging

In the last month my system has been constantly hanging. It has been slowing down for the past six months, but now certain programs wont even open ( Outlook) and most become unresponsive for varying lengths of time. I have tried to repair Outlook and my PST/OST files, and even the repair process crashes
I have checked for virus/malware with Norton and Microsoft Sec Essentials, followed loads of the troubleshooting advice, de-frag, disc clean etc, and nothing has made any difference. All on board performance analyses show nothing wrong?

In anticipation of a hard drive crash I have been trying to back up to a hard drive, which in itself is taking a very long time? ( 23% after 24 hours)

Any thoughts or advice would be appreciated.