Have you tried just plugging the HD in to the mac. When I converted to mac (G-5 Power Mac) from a Microsoft based PC, I just plugged my Seagate portable HD into the firewire port and had iTunes import all my music.
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Vegasears is right. If your Mac was built anytime in the last few years it will have both USB and Firewire ports built in.
If your external hard drive has either of those interfaces, you can simply plug it into the Mac using the appropriate cable and it should mount on the desktop and also be visible in a Finder window, even if the drive is formatted for a PC rather than a Mac.
You then have the choice of using iTunes to import the music files or you can simply drag the files from the external hard drive to the internal drive of your Mac to transfer them.
iTunes directly supports these file formats: MP3, AIFF, WAV, MPEG-4, AAC, and Apple Lossless. If you ripped the files into some other format you will need either a plug-in for iTunes or an application to convert them.
The other two posters are correct in that you can most likely simply move the drive from one machine to another.
However, if you have two computers, you can benefit in the long run by having them networked. For one, you can share an Internet connection. Assuming they are reasonably close in proximity, get a 4-port ethernet switch (around $40) and 2 Cat 5E patch cords of the appropriate length. If you connect to the Internet via a dedicated router, you can point both machines to it to automatically determine network settings. Otherwise, you can set up your network manually by giving each machine what is called an IP address (use one that is dedicated to private network use, such as anything starting with 192.168).
It's beyond the scope of this forum to explain exactly how to configure the network, but you can access and change that information on the Windows box with Settings->Control Panel->Network Connections, and on the Mac with System Preferences->Network.
There is a ton in infomation on the web about how to network computers for home use.