Yes, you can add a 2nd Hard Drive to your system (I have 4).
Installing it is fairly easy and most boxed Hard Drives come with good instructions on how to do it.
You WILL have to format it though as the drive manufacturer does not know what OS you are using MAC, Linux, Windows 9x
or NT. Its really easy to format though and in fact you can have
your system (Windows XP) format it for you. XP in fact recognises the drive on startup and walks you through the process of formatting. Its really painless!
The only trick is getting the right kind of drive for your motherboard. Unless you have a SCSI card, your first Hard Drive is an IDE. IDE's are easy to daisychain and you allowed to put two devices on each channel (cable).
Since a T-bird 1.2 is fairly recent as processors go, you have a Motherboard that supports most of the IDE Hard Drives out there.
I'm guessing that it is IDE-33 or IDE-66. Sound familiar? Does your computer come with specifications? Looking at you current Hard Drive will also work in a pinch.
What Hard Drive manufacturer to purchase and where to get it
are your choice. But Maxtor makes good drives and are pretty easy to find. IBM (my choice), Western Digital, and Seagate are others.
So for now do a little research, go get a good deal, and if you get stuck on installation...post here again and I or others will help you
the rest of the way.