C# does have a big enough data type to hold big decimal numbers.
So, coding will not help.
After googling a lot I found a website to do it for me:
Luckily, I didn't need this in a program. I just had to verify some numbers. When you pull up the certificate properties in the "certificate repository", it shows up in hex. But in my "Signed Xml", the serial number showed up as decimal. So, i just needed a tool to convert between the two.