Acer Aspire one Netbook spare or repair +

For sale is my Acer Aspire one Net book which my son has been using but it now comes up with an error message when Its switched on and wont load the software - the error is UNAMOUNTABLE-BOOT-VOLUME ?? It was running windows XP and was perfect for surfing the web etc. The model number is ZG5 if you want to check the full spec . Im sure if someone has a USB disc drive then once an operating system has been installed it will work fine again but at the moment it is being sold as spare or repair. It comes with the charger.