Is it a trojan horse causing problem?

I have windows XP media centre (up to date with all updates till support dried up from microsoft) and Internet Explorer 8. I am aware these not now suported by Microsoft.

I am up to date with Norton internet security and do frequent scans.  I also scan with the free version of Spybot Search and Destroy.

Problem - I am now fairly frequently getting Norton internet security blocking I.E. from accessing some websites because they are high risk.

The attempt to open these bad websites seems to be happening without any action from me, ......I.E.  just tries to connect of it's own accord even while I am doing something else on the computer.

A common denominator seems to be that the web site is    http://static.    followed by  11 seemingly random letters then .com then lots more letters or digits (probably hex?).

It would seem that there is a Trojan Horse on my PC somewhere?

Can anyone help rid me of this problem?  I am pretty low down the knowledge scale on computer working so any tech language is fairly uninteligable to me, made worse by my advancing years and failing 'short time memory' ability.  So I need simple instructions.

I have just bought a macbook air....now my head hurts even more getting to grips with the OS but am getting there slowly!

I will eventually get a replacement desktop as mine is not updatable to later Windows OS (Dell dimension 5150) but need to continue PC use till all files moved to laptop (more head hurting?).  Will probably get a Mac desktop.

Apple shop seemed  to think an antivirus software was not required on Apple computers??  is that true?

Thanks in advance for any help.