Do you have a firewall? If not, least you can do is plugin a network router.
Change all the local administrator accounts password.
Patch the Windows to the latest.
Pull out the command prompt by "start" --> "run" --> cmd
In the DOS prompt, type the following.
netstat -a 'this will provide information on what ports are listening. Check for port 20 or 21.
Also do nestat -b ' this will provide information on what executables are running on what ports
Check services.msc on the run to see if anything out of ordinary running. If you find certain executables that were non-business related, stop the service from starting automatically.
Also check any executables that are loaded in your system memory by task manager. Google all the executables and processes that you are not unsure of.
Once you identified the executables from task manager, find the executables from your HDD, then rename them to your own extension, e.g., .zzz. This is for you to find and delete.
Check the startup. You can search through the registry, or use utility like msconfig or startup (http://www.mlin.net).
After you performed all the tasks and a reboot, confirm with netstat -b.
I would also do the Virus scan to eliminate any stealth or trojan horse.
Cheers.