|Dell doesn't make hard drives, they don't make anything! They're a builder/assembler, not a manufacturer.|
You didn't post the model/model number of your system so it's impossible to tell you what to get. There are 2 common types of HDDs - IDE (aka PATA) & SATA. We'd need to know which you have. And depending on the age of your system, it may not support HDDs over 128GB.
When you buy a new drive, it's completely blank. It will have to be partitioned, formatted, & Windows will need to be installed. You will also have to install drivers for all your devices (video, sound, network, etc), you will need to run Windows Update, & you will need to install an anti-virus program & whatever other software programs you use. And if your system has a SATA HDD & you want to install XP, it gets even more complicated.
Start by posting the model/model number so one of us can look up the specs.