wchar *s; // эта строка
wchar  c1, c2;
int  n;

n = wstrlen(s);
for (i=0; i<n/2; i++)
{
    c1 = s[i];
    c2 = s[n-i-1];
    s[i] = c2;
    s[n-i-1] = c1;
}