If I reformat my harddisk and re-install the same cd key of the windows xp that i purchase?

When you activate Windows you send two things to Microsoft - your activation key and a "fingerprint" of your computer. If it's the same computer, you can activate as many times as you like. (If you change enough to change the fingerprint, you just call them and they remove that key-fingerprint from the database, the old configuration is no longer "genuine Windows" and you can activate the new configuration.) Your ONLY problem, if it's a Windows installation disk, not one from the computer manufacturer, is that it won't have all the drivers you need. Before you format the computer, go to the manufacturer's website, download all the drivers for your computer for your version of Windows, and save them to a flash drive or burn them to a CD. Then you can install them after you reinstall Windows. (If it's just a Windows Repair CD from the manufacturer, it won't install Windows, it'll just repair an existing installation, so make sure what you have. If it's an installation disk, let the ...  more