hi to all ,
these are good resources for NN library in C and C++:

1-FANN library by Steffen Nissen (Fast Artificial Neural Network Library)
Fast Artificial Neural Network Library

2-LWNN library by Peter van Rossum (Light Weight Neural Network Library)
SourceForge.net: Lightweight Neural Network

3-A good hardware acceleration for large computational application:
by using GPU multi processing threads (NVIDIA CUDA , AMD ATI...)
Nvidia is provided a good C library for using GPU power in your application
a comparision:
(Intel Core 2 Quad Extreme QX9770 : less than 100 GFLOPS (single 32bit float) or (double 64bit float) )
(NVIDIA GTX260 , GTX280: 933 GFLOPS(single 32bit float) and 100 GFLOPS (double 64bit float))
CUDA Zone -- The resource for CUDA developers