I have been trying to get data off from 2 old drives…

I have been trying to get data off from 2 old drives. I have placed them in HD enclosures and have plugged them into the USB port and the computer states that the hardware is installed and ready to use, but they do not appear in the My Computer section or anywhere else. I tried the Computer Management and the only option that is given me is to delete the partition. These 2 old HD’s have the good old Microsoft Millennium system on them and they were the original systems drives (C Drives). I need the data; what can I do if anything?

Thanks in advance
h