Adding a second hard drive.

If your computer does not have a slot big enough for the second hard drive I would recommend a few things. 1st an external hard drive. You could buy a new case since it is a micro case and thus pressed for internal space. However I do not recommend this. The cost in relation to the value of the computer is too high. You could exchange the present hard drive (standard was 40GB). Alternatively a card reader would up your storage space relatively cheaply. Since it is from 2004 it is reaching the end of it's life cycle and getting a new one would be advisable. There are many relatively cheap options with the same or better performance and if you are keeping your monitor this will keep the cost down. if you insist on buying a case try places that build them on-site as they may have old cases for cheap or what's called open box cases. Literally you will not get a case or any manual.

Are we talking about external HDD?, If Yes, It is called as the HDD Enclosure, you can buy it in any computer shops out there..If we are talking about internal HDD, I believe there's already a slot inside the CPU where you can just screw-in the new HDD.