You will need:
Portable hard drive to back up all of your files.
Windows installation disc
Installation discs for any software that you have on your computer such as Microsoft office.
The key to this is to back up all of your files. You will lose everything after you format your hard drive so anything not backed up will be permanently lost. A portable hard drive should allow you to easy back-up all of your photos, music, videos and documents onto a single hard drive.
Once all of your files are backed up, reboot and press f8 until the boot menu appears. Insert your windows installation disc and select boot from disc. You will want to format your main partition then reinstall windows.
Follow these instructions from microsoft to format your partition: http://windows.microsoft.com/en-US/windows-vista/Create-and-format-a-hard-disk-partition
Next, install a new version of windows. You will need a valid license key to proceed.
Finally, reinstall all software on your system and put your backed up files back in place.
Your system will be good as new.