I have an old polar RS800cx watch, but cannot find the Moschip MCS7780 USBFIR adapter #2 driver for the USB IR dongle.
If you're still interested, You can try this in Windows 10:
You can try searching for the Moschip MCS7780 USBFIR adapter #2 driver on the manufacturer's website or on driver download websites. You can also try purchasing a new USB IRDA adapter that is compatible with the Polar RS800cx watch.