A hard drive from another computer can be used in another computer usually without any problems as a secondary drive (or what is also reffered to as a "slave"). Your primary drive that you boot Windows XP from will be your primary drive (or "master").

A little bit of preparation needs to be done before you put the second hard drive in, however you don't neccesarily have to wipe the drive clean to use it as a secondary drive. If you don't need any of the information stored on the secondary drive I would recommend wiping it clean.
To wipe it clean you can use third party software or you can do a basic "format" of the drive. The easiest option is to format the drive, it basically just means that your hard drive will have a small amount of space that is not accessible (and we're only talking a few megabytes)......Another reason to format the drive is that Windows98 ran on a filing system calles FAT32. Since then Microsoft has developed a different type of filing system called NTFS. NTFS is superior to FAT32, it is also not recommended to use FAT32 with Windows XP. Therefore, in the process of formatting the secondary hard drive we can change the filing system to NTFS.....

If you are keen to give it a try, you need to open up your box and ground yourself (just touch the metal part of the box for a couple of seconds to release any static electricity from your body). Now look at the ribbon that goes into the back of your current drive, that ribbon should have another section of ribbon that has an empty hard drive connection on the end of it. If so this is where your secondary hard drive can be attached.
However before you go and attach your second hard drive, we need to ensure that the "jumper settings" are in the right position. The jumper settings are controlled by a tiny plastic plug on the back of the hard drive (right next to where the ribbon slots in). Different drives require this tiny plug to be in certain positions depending on whether the drive is being used as a "master" or a "slave". This tiny piece of plastic is vital, as it basically tells the computer on start up exactly what to do with the different hard drives. Overlooking this step or getting it wrong will cause headaches, nothing drastic it just causes a lot of mucking around.....

If you are keen to do this, look on the top of the second hard drive. It should have information about the jumper settings....if you like, post a picture of what it looks like and I will try to explain where the jumper needs to go. Aso, if you could ascertain what type of drive your current drive is (i.e manufacturers name and the code) that will be handy. We also need to make sure the jumper settings for this drive is correct in a "master"/"slave" configuration.

I will be busy with work over the next few days but I should be able to help within a day of your reply. I look forward to hearing from you.....