Trojan or hijacking - computer seriously ill!!

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Hi

I seem to have a trojan or been hijacked or something - messages keep popping up on my computer telling me that. My computer just keeps freezing and I can barely get the google homepage up at all. I'm trying to download spybot but so far I've been trying for 48 hours - my computer just won't work.

I have McAfee and did a scan and it said my computer and all the files were fine. I think the problem is in Microsoft Internet Explorer. I have no idea how to sort this out as the internet won't work. Any ideas?

Thanks,

Clinton