Excellent - I have removed my post and am seeking Hormone Replacement Therapy
Now then.
You use a router? Any software firewall, Zone Alarm, etc?
Usually CRCs are sourced from things like application layer firewalls on routers or on your PC helpfully trying to screen traffic to make sure it's ok and in between breaking it.
Try booting in safe mode with networking from behind the router. We know it isn't the router as you've already tried directly connected with no change.
To eliminate network corruption pop open a command prompt and do this:
ping -t -l 1460
www.virginmedia.co.uk
Leave it running for ages, and when you get bored hit CTRL-C and see if any errored.
You can also do the Mac equivalent if you wish too.
ping -fs 1460
www.virginmedia.co.uk
Don't leave the Mac one going for too long as it is a flood ping and looks like a denial of service