Cant back up because of unknown error (-20) - OS windows 7

Please help!

Everytime I try to do anything that involves backing up my iPhone 4, this message comes up: 'iTunes could not backup the iPhone "Jack's iPhone" because an unknown error occurred (-20)'

I understand that to solve the problem i may have to delete/move the last backup file but i dont know how to do this on windows 7

Thannks in advance,
Jack

Thanks alot done it and works perfectly