Should label and field be separated with colon?

When you have a form with fields and captions for these fields, should you do it like that:

Field1: [                       ]
Field2: [                       ]

or like that:

Field1  [                       ]
Field2  [                       ]

I saw both used in popular application. Microsoft generally uses colons (My computer properties -> Computer Name tab is one example). Opera browser doesn't use colons as separators on forms (example: Preferences form).

I personally don't have a strong opinion on which one is better. But I'm working on this "1000s of winforms" app where I suspect we have forms with and without colons so I would really like to set a standard and go one way or another on all forms because the inconsistency is far worse than any of the options.