When you say the system is 'Set up for PC', what do you mean? To play music out of a PC you'd be plugging in an audio jack like this: https/images-na.ssl-images-amazon.com/images/I/31KyWV-SFkL._SX355_.jpg and you'd do exactly the same on an older Macbook - it would just go into the headphone port.
When you say the system is 'Set up for PC', what do you mean? To play music out of a PC you'd be plugging in an audio jack like this: https/images-na.ssl-images-amazon.com/images/I/31KyWV-SFkL._SX355_.jpg and you'd do exactly the same on an older Macbook - it would just go into the headphone port.
Speakers themselves are not OS specific --- you should be able to use them on any computer or laptop provided there's a compatible audio jack on that particular PC/laptop, whether it be a generic Windows PC or a Mac is irrelevant.