MacAir Safari loading problem

I have a MacAir 1.86 GHz Intel Core 2 Duo running OS X 10.7.5 with 26GB free and 86GB used on the Hard Disk. I am having serious issues trying to open Safari which usually results in the spinning disk for ages. Quitting and re-starting used to work but even that usually fails now. My iPhone using the same home wi-fi works fine. When I click the wi-fi "fan" on the laptop it always shows "Looking for networks" then seems to find the home network but still doesn't open the internet reliably failing more often than succeeding. This has got progressively worse in last 2-3 weeks. Can anyone help?

Troubleshooting tried: according to Omni Disk Sweeper: 78GB of the Hard Disk is "Others" of which 64GB is "Users" of which 27GB is my Mail file (which I prefer not to delete).

I'm not technically smart so layman's language please!

Thanks David