RIS stands for remote installation services. It is used to install image from a Window server to a new hardware. We use RIS because installing OS every time using a CD would be more time consuming.
You need to re-install another system, latest anti-virus software and an O.S with latest patches. Before you start your system, connect the infected HDD as a secondary drive then scan and clean the infected hard drive. Once done, you can copy the files into the system.