The driver for your NIC may not support changing the MAC address. It all depends on how you are trying to change the MAC address though. If you are trying to "permanently" change the MAC, you'll need to get your hands on the tool your motherboard manufacturer used to program the MAC on the production line. These tools are not usually published by the NIC/motherboard manufacturers.
If you only want to change the MAC temporarily, most NIC drivers do support making a change. You may be entering the MAC in an invalid format or the driver may not support making a change. As far as the change being rejected because the MAC is already in use on your network - that won't happen. You can have duplicate MAC addresses on a network, you will just get weird problems, but your OS won't validate the MAC before applying it.
Depending on the type of spoofing you want to do, you might try something like Cain and Abel.
HTH