Is your computer from a name-brand manufacturer (Dell, HP, etc?) If so they
often come with either a system restore CD or have an option in the Start
Menu to restore to defaults. For more information, contact your computer
vendor as specific instructions will vary by make and model.
If it was a system built by a friend or the local computer shop then you'd
back up any important information, pop your Windows CD into the CD-ROM
drive, reboot and boot from that CD to start the installation process.
Joe