Backup documents, music, etc.
Insert Windows Disc
Go to BIOS and alter boot order to boot from CD first.
Install Windows again, over the partition that it is currently installed on.
Wait 45 minutes for it to install.
Wait 90 minutes to install updates from internet if you have broadband.
Done and Done... install anti virus and anti malware/adware software. Keep it updated.

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1985 CA 300D Turbo , 213K mi