hi
You have two choices, either you use Reflector's addin http://www.denisbauer.com/NETTools/FileDisassembler.aspx, which is free, or commercial ( and pricy) .net dissemblers such as http://www.9rays.net/Category/53-spices_net_suite.aspx or http://www.remotesoft.com/salamander/.
I used Filedisassembler; the quality of the decompiled code is very bad. The decompiler code cannot be recompiled.
I'm sure about the quality of commercial decompilers but I suspect that it would be much better.
Edit: To use Filedisassembler, go to Reflector, View->Addins, click Add and select theFiledisassembler.dll.