Large number arithmetics (Decrypting RSA signature)

[Andrus Moor]

I have a RSA signature S  (128 bytes binary data )  ,
modulus n (also 128 bytes binary data)
and exponent e (which has value 3 or 2**16-1 only)
I need to calculate
( S**e ) MOD n
All I need is to perform exponent ( S**e ) and MOD operation on 
128 byte integers.

Windows CryptoAPI CryptDecrypt() returns invalid key error message.
How to calculate this ?
Any sample code ?
Andrus.