MacBook Pro, Lion and slow internet

Question

I upgraded to Lion yesterday to see if it resolves a persistent slow internet connectivity problem but unfortunately to no effect.
Symptoms are:
slow connectivity / download on both wireless and wired connections.
Compared to my Alienware Windows laptop, the MacBook achieves less than 10% of the downoad and upload speed in either state
Speed has been checked on many sites including speedtest.net  - practically, connection is sluggish for both web and non-web internet use.
Ping to sites is identical on the Macbook Pro and my windows machine. There is no drop of ping or excessive variation.
The issue is independant of browser - Firefox, Safari, etc
I have a BT 40MB infinity connection at home and a 10MB uncontended leased line connection at work - the issue exists wired or wireless in both arenas
I can achieve download speeds of around 38 Mbps on my Alienware, wired, and 17 Mbps wireless. This compares to less than 2 Mbps on the Macbook Pro
The condition started under Snow Leopard about 4 weeks ago - unsure if it was after a routine upgrade
Following advice on these boards I have tried changing DNS to various open DNS, with no success
I have tried altering the mtu settings as directed, with no impact
The issue is therefore independant of router, internet provider ( I have access to three), wired or wireless connectivity. It also preexisted the Lion upgrade
I know there are other, similar threads on here, but my symptoms are different (no drop of ping, not just a wireless issue, multiple networks)
Any suggestions or recommendations would be welcome.
Cheers

Answer 1

Thanks for advise but now resolved.
Being paranoid and from a Windows background I had put an anti-virus program on - Sophos. Frankly I had forgotten about it. Turing it off resulted in a 1000% improvement in my internet speed. Who would have known?
OK I should have guessed a little earlier.
Cheers