You can reinstall any version of Windows on a machine as long as it was previously installed and activated on that machine. Meaning that you can use the same serial for the XP install again that you used previously. You wont have to buy it again. But you will have to use the install media to reinstall XP. If the Acer netbook came with XP preinstalled and has the serial sticker on the bottom, you will need a OEM version of XP to use that serial again. If you dont have the media, contact Acer to get it or you can try this page from Microsoft
If you installed XP yourself you will just need to use the same disk as you did previously.