Depends what you mean by saves, and what OS you are using. With windows 10 you can often just leave windows installed and connect the drive and it'll boot up and find new hardware. Wether or not you can activate it again depends on what licence you have and there are no guarantees that this method will work so you should back everything up first. A clean install is always better though.
If you mean game saves you need to look up the individual games for how to transfer them. Steam can just be copied.