|I suggest you contact the local authorities cyber crime division and report that. They can advise you what to do. They may want to examine the computer before wiping it.|
There is a program called killdisk that can safely wipe the hard drive. If this is a computer that came with Windows pre-installed hopefully you have a set of restore disks.
Are you a business or home user?
Depending on what type of sensitive information was on the computer you need to take steps to secure any bank, credit, or other type accounts. Any passwords you had stored on the computer should be changed, using a different computer.
That could happen remotely as well. That is why I don't store ANY sensitive information on my computers.