You will lose all of your data. Make sure you have good backup before you start. Have your drivers disk ready. The first thing you want to do, after the wipe, is make sure all of your drivers loaded. If you dont have the disk, your computer manufacturers website should have them avaialbe for download. If you have any USB devices make sure you install the driver before the device! If you have an external hard drive of flash drive unplug them before you wipe out the machine. The very next thing you want to do, after you have all of your drivers, is go to Windows Update and get all of the updates that are available for your computer. You will have to restart a few times but just keep going back untill it stops finding updates.
When you do the install make sure you follow this order:
1.Delete partition
2. Create partition
3. Format your partition (dont use the the quick method)
If you are installing XP this site explains it pretty well:
http://www.michaelstevenstech.com/cleanxpinstall.html
