I found this bit of code on the Microsoft Community page that should work in the Field Calculator using VBScript:
Assuming there is no space character in the data:
Replace( LTRIM(Replace( fieldName, "0", " ")), " ", "0")
The part in bold replace all zeros by spaces, and then, left-trim that result, ie, remove starting spaces. The external Replace replaces the spaces left in the string to their initial 0 character.