Older macs don't do so well with all the possible windows disk formats. Some may for example only read and not be able to write. But in most cases they can read fine.
The most common problem is the jumper on the drive for master/slave/cableselect. The enclosures usually prefer either master or cableselect. Sometimes you have to experiment. I've ran into a few specific cases where the jumper had the three possible positions, and required me to REMOVE the jumper entirely (nothing selected) for it to work.
If you're using a drivemate, bytecc, or one of the other dongleish units that just plug onto the bare hard drive, there's usually a very specific order and sometimes some timing you have to follow when plugging in cables to get the drive to show up. I know a couple people that never were able to "get the trick to it".
We need to know more information about your setup to help further. What mac os version? what do you know about the hard drive's partition scheme, and its partition type? what enclosure/adapter are you using? What order are you connecting things? what else have you tried? tried hooking it up to a windows computer over usb?
