Is there a way to view the body / contents of a system function in sql server 2008
I don't think Sum is stored as a T-SQL function in your system that you could "peek" into. That's most likely code deep inside the SQL Server core.

As a proof - you can e.g. get the body of other system stored functions using sp_helptext:

sp_helptext 'sys.fn_isrolemember'


That works just fine - so that leads me to believe that Sum isn't a stored function as such.