Answer:
boot the computer from the windows installation disk and run startup repair
Step-by-step explanation:
Based on the scenario being described within the question it can be said that the best option in this situation would be to boot the computer from the windows installation disk and run startup repair. This will analyze all windows files and clean up any files or malicious software that does not belong in order for the system to be able to correctly boot into the operating system. Then from inside the operating system you can run a full diagnostic of the system.