Use the str() builtin, which:

Returns a string containing a nicely printable representation of an object.

E.g:

>>> import decimal
>>> dec = decimal.Decimal('10.0')
>>> str(dec)
'10.0'

Hmm... I can't reproduce this:

using System;

class Test
{
    static void Main()        
    {
        decimal d = decimal.Parse("1200.00");
        Console.WriteLine(d); // Prints 1200.00
    }
}

Are you sure it's not some other part of your code normalizing the decimal value later?

Just in case it's cultural issues, try this version which shouldn't depend on your locale at all:

using System;
using System.Globalization;

class Test
{
    static void Main()        
    {
        decimal d = decimal.Parse("1200.00", CultureInfo.InvariantCulture);
        Console.WriteLine(d.ToString(CultureInfo.InvariantCulture));
    }
}