The Windows XP CD included with most computers shipped with the operating system already installed acts as a boot CD . This CD allows the user to repair, reformat and reinstall the computer's operating system. It is advisable to reinstall the operating system every now and then to clear out the sludge and speed everything back up a bit. If you've lost the Windows XP CD, you might be at a loss as to how to preform this operation, but Microsoft actually includes the Windows XP installation files on the hard drive. Accessing the proper files, you can reinstall the system as if using the CD.
Back up all data to an external hard drive before proceeding.
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Open the "i386" folder within "Windows" located in the root of the C: drive.
Double-click the "winnt32.exe" file. This will launch the Windows XP installer utility.
Follow the on-screen prompts for instructions on how to format and reinstall Windows XP. Enter your Windows XP Product Key as prompted (located on the computer itself, most likely). When the process completes, the system will reboot and launch into your newly reinstalled Windows XP operating system.