Euler

I'm working on Euler problem number 12. I've got the following:

//cut out my includes
bool IsPrime(int num)
{
    bool prime = true;
    int number2 =(int) floor (sqrt ((long double)num));
    for ( int j = 2; j <= number2; j++){
        if ( num!=j && num % j == 0 ){     
            prime = false;
            break;
        }
    }
    return prime;
}

int NumDivisors(__int64 num)
{
    int count = 0;
    if(!IsPrime(num))
    {
        __int64 number2 = num/2;
        for ( __int64 j = 2; j <= number2; j++){
            if ( num % j == 0 ){     
                count++;
            }
        }
    }
    return count;
}

int _tmain(int argc, _TCHAR* argv[])
{
    bool found = false;
    __int64 num = 1;
    __int64 maxdiv = 0;
    while(!found)
    {
        __int64 trinum = 0;
        for(int i = 0;i<=num;++i) trinum += i;
        int div = NumDivisors(trinum);
        if(div>maxdiv) maxdiv = div;
        if(div>500) found = true;
        if(!found) ++num;
    }
    return 0;
}
The problem is, NumDivisors is horribly inefficient. I mean 30 minutes at 2.5ghz and I'm only on num=9219 with a maxdiv of 478. Any suggestions?

There's a faster algorithm to get the number of divisors. You only have to check up to the sqrt of the number, but for every divisor in that group, theres actually two real divisors: the number itself, and num/divisor, or something to that effect.