.NET, you can put quotation marks around a string like this:
Dim strTest = """" & stringToQuote & """"
(Note the two sets of four double quote characters!)
In C#, do this:
string strTest = "\"" + stringToQuote + "\""
\" is the escape sequence for a double quote and (of course) you have to surround it with double quote characters as well. So we have three instead of four.
If you do that with the string before appending it to the url, it should solve your problem.