You need to check with Acer support and see if they offer Windows XP compatible drivers your Acer notebook. If they do, then you need to boot from the Windows XP CD, delete all partitions, create a new partion, format the new partition and install Windows XP. Afterward, you then install all the Acer-provided Windows XP hardware drivers.
If Acer does not offer Windows XP drivers for your particular Acer notebook, then you cannot downgrade since you'll not be able to use many of the notebook's unique hardware devices, such as audio, NIC, wireless, etc.
