Before doing anything , Back-up all you personal files, either to the cloud, or USB/DVD for more secured info. and make a list of the software you may want to reinstall, (Saves on the "what was the name of that program?" problems.)
First and foremost check your license key sticker on the outside side of the computer, to make sure you can clearly read all digits of the key.(Smart phone users may want to take a picture of the sticker so you can zoom in and read it better) if not download a license crawler program to locate the windows XP key the system was activated with, and copy it down. you can't reinstall with out it.
Next go to computer manufacturer's web site, and locate the device driver's for your specific computer(Some sites do have an auto detect feature that makes this very simple with a working machine). Download ALL the device drivers listed since there is no way of knowing what drivers XP will be missing, and most often it's the network driver, making it difficult (if not imposable) to get these later. At the very least grab the Network, graphics/video, Audio Drivers, as well as the Chipset, and BIOS Installers.Save these on Disk or USB so you can use them later.( I save each one in separate folders since files names won't always tell you what they are.)
you will also need to download a bootable partitioning tool and burn it to disk (or bootable USB works if your BIOS allows USB boot). I like Parted magic, it can wipe, and reformat a drive in just couple minutes( around 40 seconds for an 80 Gig HDD).
I assume you already have your XP install disk( or disks if Media Center XP). If not these can be found and downloaded on-line. Make sure you have/download the proper version on your sticker,i.e... Home, Pro, Media Center.
once you have all three items,
1. bootable Partition editor
2. install disk w/ license key
3. Drivers saved to Disk/USB
you're ready to start.
reboot your computer using the bootable partition editor ( you may need to tell your BIOS to boot that drive first, default is usually CD, then HDD (now blank), So that some times USB (if available on the system) doesn't get a chance to boot unless told too.
Delete, and reformat the HDD ( I like NTFS format,but FAT32 works as well).
USE CAUTION!!!!
Some Programs offer (and default to) formats for Linux, or Unix, that window can not even see.
You may need to create a partition table if your software does not do this automatically.
then you can reboot to your install disk and let windows work it's magic. inserting the license key when asked for.
when the install is completed and you can see your desktop, right click on "My Computer" either on the desktop, or in the start menu, and select "Properties" at the very bottom of the list. then find the "Hardware" Tab, and select "Device Manager". This will tell you what drivers are missing, anything with a big yellow "?" is missing it's driver, anything red is a more serious matter, (like hardware failure). install the necessary drivers from the ones you downloaded earlier.( Hint: if you install in the order of; Chipset, Drivers, BIOS, you can select reboot later on every thing, the BIOS will force the reboot for every thing, saving you loads of time in restarts.)
connect to the internet, and activate windows, then install an antivirus program. (Microsoft's Security Essentials works well, and is offered to all windows user's for the low low price of free, and updates with Windows.)
Now your ready to do your updates, even with a good connection this can take some time, and multiple reboots, (sorry, can't help with that, Microsoft has updates for the updates.) always select custom on the update page, the other only gives the high priority, and may not help to update your firmware. continue to run updates until all sections of the left panel reads 0.
CONGRATULATIONS!! you now have a clean, activated, secured, and updated windows XP operating system, ( Providing all went well.) ready for whatever you want to throw at it.( I prefer a Brick 
Hope it Helps.