As of .NET 4.0 you can use the System.Numerics.BigInteger class. See documentation here: http://msdn.microsoft.com/en-us/library/system.numerics.biginteger(v=vs.110).aspx
Another alternative is the IntX class.
IntX is an arbitrary precision
integers library written in pure C#
2.0 with fast – O(N * log N) – multiplication/division algorithms
implementation. It provides all the
basic operations on integers like
addition, multiplication, comparing,
bitwise shifting etc.