What OS is on each. Also, a straight serial cable is not going to work. You need ones designed for communications that have the transmit and receive lines reversed so the XMIT of one connects to the RCV of the other. These connections tend to be very slow. You can get parallel communication cables that will be much faster. At least from Win98 up, no special software is needed. A communication package called LapLink was once used for your purpose. I am not sure if it still exists. It will have more features than the OS native utility can provide.
Warning: remedial computer person here. I'm trying to connect an old cpu to my laptop so I can use its hard drive. I've connected it physically (with a 25-pin serial), but I don't know what software I need to get, or if I'm even going about this the right way. Any guidance would be most appreciated!