Help - An unknown error occurred (-50)

For the last few weeks I have been getting 'An unknown error occurred (-50)' when trying to download new purchases or updates via iTunes, can anyone help?

I've tried every solution I have encountered all with no success. Off the top of my head I've checked the firewall and antivirus, tried deleting partial downloads in the iTunes folder, uninstalled the one download accelerator I had, tried a winsock reset, uninstalled and re-installed iTunes and still no luck. This is driving me nuts, I can download everything fine on our iPad or iPhones and then sync these to our laptop but virtually any file over 20Mb seems to fail with the above error when downloading on the laptop. I'm running iTunes on Windows 7 x64.

Any help much appreciated.