Browser Hacked?

I am using a MacBook running system 10.6.8

The computer has been behaving strangely for some 2 months, the following is an outline

a) A Friend said he received an e-mail from my website e-mail with the title: 'You have been Hacked'.

b) Some emails sent from one registered address in Mail arrived at the destination with a different e-mail address (but one that is registered in mail)

c) A drop down in Safari from api.mywot.com/requested a user name and password, the browser wouldn't work and a reboot eliminated the problem.

d) Switched to Google Chrome.

- ISP checked modem and found no issues

e) When clicking to change the page in my Youtube channel it redirected to the browser page that accesses the router/modem.

f) While watching a program of Filmon.com, a chat session started within the video screen; I found chat-services.filmon.com may have caused that - this connection is now denied in Littlesnitch.

g) Also in Youtube the top part of the browser screen changed colour with a cartoon of a sleuth character in the top right. When I reloaded the screen the browser window returned to normal.

- Recently the ISP changed to new router/modem

h) Again in Youtube the top part of the browser screen changed colour with a cartoon of a sleuth character in the top right; as before.

i) No other issues have happened.

Numerous Scans with Sophos, ClamXav and MacScan all din't find any issues.

I realise these issues could be a browser hack to access the network - any ideas on how to resolve the issues, or how to ensure the computer is now issue free would be most welcome. Thank you.