Re-install completely from scratch using the installation disks that came with the computer. This is to bring it back to new factory state as like when it was bought. If it will not let you, remove the hard disk, add it to another computer using USB adapter, and remove the partition(s) on it. When putting it back to the new computer it will be blank. Run the factory installation disks or USB key to re-install as like new. You may need the registration key. Should be with the documents that came with the computer or on a tag that is with the software.