rebuild it - most likely they've installed something on it for remote control.
Rebuild and also most important DO not log into important things from this computer if known to have some form of virus. Reevulate what site's and downloads you have done so it does not happen again. Install a virus protection.
If you had a wallet file on that computer create a new wallet on another computer and send coins to it. MAKE SURE that computer does not have the same software you talk about on miner.