This site is dedicated to malware removal issues. As such, it is not a troubleshooting site, nor a site that deals with hardware issues. However, we will do our best to steer you in the right direction where you can get the help you need.
As you have a Dell computer, you will probably find a wealth of information on their site. Go here to locate a service manual for your Dell desktop computer, which should provide you with instructions for opening the case and removing/replacing the hard drive, etc.http://support.dell.com/support/topics/ ... ~ck=anavml
The best way to get information specific to your computer is to locate the "Service Tag," which is a small sticker located on your computer's case. It can be on the top, the side, or the back of your computer, but it will be there unless you have removed it.
You should be able to determine the maximum hard drive size appropriate for your computer on Dell's site, as well as the type of hard drive needed, e.g., IDE or SATA. If your computer is more than a couple of years old, your hard drive will be more likely to be connected via an IDE cable (broad and flat, usually gray in color) rather than a SATA cable (narrow, flat and rather rubbery feeling, usually red in color with small black connectors).
Dell has community forums where you can register and ask questions relative to your particular Dell model. There is one that deals specifically with drives:http://en.community.dell.com/forums/3534.aspx
Your computer has an unusually small hard drive, so I'm assuming it may be quite old. If that is the case, you should probably weigh the cost of upgrading it against the cost of a new system, especially since new computers are selling rather cheaply these days. The choice is yours. Keep in mind that if you do not have the original installation disks for your operating system, you will need to add the cost of disk replacement to the cost of a new hard drive. Also, when you're finished upgrading, you will still have an old computer with older and slower components than are on today's faster computers.
The link below is to a basic general video tutorial on installing an IDE hard drive. Although it goes into a great deal of detail on installing the cable, you will probably not be disconnecting your cable from the motherboard, but will just detach the IDE cable from the old hard drive and reattach it to the new hard drive, and the same for the molex power cable. Your IDE cable will probably not look like the one in the video as your IDE cable will most likely be broad and flat, not rounded.
Pay particular attention to the instructions to disconnect everything from the computer, especially the power cord, and to ground yourself or wear an anti-static wrist band. Even a very minute amount of static electricity can destroy computer components. If possible, stand on a hard uncarpeted surface while working inside your computer. http://www.youtube.com/watch?v=bt7ve7Ie ... re=related
You will need to locate your installation disks in order to install Windows on your new hard drive. You will need the XP Home installation disk, and you should also have a disk for installing the computer's Drivers. You may also have a disk for various applications that came with your computer. The XP disk and the Driver disk are the most important disks. If you do not have these disks, you should be able to purchase replacement disks from Dell at a reasonable cost.