One option is to use a USB-to-RS232 interface. Make sure it's RS232, not just serial (aka, serial TTL). The voltages are completely different. If one end is USB and the other is DB9 or DB25, it's likely the right one.
Depending on the interface chip used and the corresponding OS, there might be an issue with the USB compatibility, at least that's what I've heard. So, if at first you don't succeed, try a different one. Also, check through the forums for related discussions.
If you want to try GPIB, I've seen a couple of projects to build a USB-to-GPIB interface. They're quite pricey otherwise. One project used only an Arduino Uno and a GPIB connector. Without the line driver chips, it won't work with really long cables or many devices on the GPIB bus, but that likely won't be an issue for your use case.