You could use some kind of SSL API like OpenSSL, but this may be overkill in your scenario as you would need to use certificates etc. There is a open source Rijndael (the algorithm that AES uses) C++ implementation here.
Here is an example of its usage:
void testEncryptBlock()
{
const int nCharacters = 16;
char szHex[33];
char *EncryptedData = new char[nCharacters + 1];
CRijndael rijndael;
int result = rijndael.MakeKey("abcdefghabcdefghabcdefghabcdefgh");
// Add some dummy data for the sake of the demo
EncryptedData = (char*)memset(EncryptedData, 255, nCharacters); // 0xfffff...
result = rijndael.EncryptBlock(EncryptedData);
... // Do something with the data
delete [] EncryptedData;
}