That computer shipped with Windows XP, and a Hitachi EIDE 60 GB hard drive.
You can replace that drive with any hard drive up to 320 GB EIDE from any of a number of online retailers... You will find the Toshiba ATA6 to be superb, as will be the Seagate and Western Digital IDE or EIDE. I would avoid the Samsung, but if you get a Toshiba, Seagate, Western Digital that is 120 GB, 160GB or if you can find it, 250 GB, you will be happy and it will last twice as long as the Hitachi did... easy to replace... just put the new drive in the caddy that now holds the old Hitachi.
There is a manual online at Dell you can locate using that Service Tag number if you need a little guidance... But you are essentially removing two screws from the case, and four screws from the old hard drive, and replacing them when done. You need a small philips screw driver for the best work... a #1 or smaller.