VS 2010 : Add Hex Values Together?

I'm trying to write a tool for our Field Engineers to help verify data they come across at customers sites. I need to be able to convert a series of characters to Hex, and then add the values of each Hex pair together, then take the last 2 digits of that sum and use it as a checksum to verify the line's contents.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim byteArray() As Byte
Dim hexNumbers As System.Text.StringBuilder = New System.Text.StringBuilder

[code]...

This takes whatever text is entered in Textbox1, and outputs the corresponding HEX values in Box2.For example, "HI" = 48 49 (output as 4849). What I need to now figure out is how to either sum the character values together, or maybe split them apart into an array and add each line of the array together(so I need to add 48+49, to get 91 in this case) I thought about using the "hexnumbers.tostring()" line to then split the string, and add each pair together, but this does not work if there are letters involved, since you can only add letters in Hex.

cliff notes version:Need to add to the above code, so that it takes text entered, such as "HELLO" which = a hex value of "48454C4C4F" and adds 48+45+4C+4C+4F, which should = 174, and then strips everything but the last two digits, so in this case "74".