First off: *never* concatenate user input; use parameters instead...
i.e. "AND ProductID LIKE @ProductID"
(and add a parameter named @ProductID with Value =
productInfo1.ProductID + "%".

However; back to the question ;-p Various options:
* A checkbox next to each that checks itself automatically when a
value is entered, but can be checked manually for searching blank
* Radios for value / blank: (o) [Textbox] (o) (blank) - defaults to
value obviously (and selecting (blank) would disable the Textbox)
* Drop-down list of available options, with (blank) at the top?
* A more dynamic search UI - i.e. user adds criteria by selecting a
property (from a list) *and* the value - and can and multiple rows -
i.e.

[Name] = Fred
[Order Ref] =
[Foo] = bar
[Select option] = {value}

(the bottom option adds a new row; the Order Ref is searching for a
blank)
This approach also: handles large numbers of searchable fields without
needing a massive search form; allows for more complex "(this and
that) or (theother)" searches; makes it easier to fit operator options
(equals/starts-with/less-than/more-than etc) without making the UI too
ugly (OK; still ugly, but not too much so).

Of course, the alternative would be to go all "google", with a single
text-box and simply parse the tokens - i.e.:
  Name=Fred and Order Ref = "" and Foo=bar
(or something)

Marc