Can anyone define the Windows PE Checksum Algorithm?

I would like to implement this in C#

I have looked here: http://www.codeproject.com/KB/cpp/PEChecksum.aspx

And am aware of the ImageHlp.dll MapFileAndCheckSum function.

However, for various reasons, I would like to implement this myself.

The best I have found is here: http://forum.sysinternals.com/optional-header-checksum-calculation_topic24214.html

But, I don't understand the explanation. Can anyone clarify how the checksum is calculated?

Thanks!

Update

I from the code example, I do not understand what this means, and how to translate it into C#

sum -= sum < low 16 bits of CheckSum in file // 16-bit borrow 
sum -= low 16 bits of CheckSum in file 
sum -= sum < high 16 bits of CheckSum in file 
sum -= high 16 bits of CheckSum in file 

Update #2

Thanks, came across some Python code that does similar too here

    def generate_checksum(self):

    # This will make sure that the data representing the PE image
    # is updated with any changes that might have been made by
    # assigning values to header fields as those are not automatically
    # updated upon assignment.
    #
    self.__data__ = self.write()

    # Get the offset to the CheckSum field in the OptionalHeader
    #
    checksum_offset = self.OPTIONAL_HEADER.__file_offset__ + 0x40 # 64

    checksum = 0

    # Verify the data is dword-aligned. Add padding if needed
    #
    remainder = len(self.__data__) % 4
    data = self.__data__ + ( '\0' * ((4-remainder) * ( remainder != 0 )) )

    for i in range( len( data ) / 4 ):

        # Skip the checksum field
        #
        if i == checksum_offset / 4:
            continue

        dword = s