Before you do anything ie format HDD, go to your device manager and get a list of hardware components that your laptop uses or a list of drivers. If you cannot find the XP drivers on any acer site ie acer europe, acer canada, acer usa etc, then you will have to go to the hardware manufacturers site and find each one.
The problem is, that special functions like media keys, hotkeys etc may not work after, unless you can find the XP utility for these (or one that works)
Suggestion would be to dual boot. Find a partitioning program that will allow you to go into your HDD and add another partition for XP.
Thisway, you will have a functioning OS (Vista) until you find all the drivers/utilities for XP.
I would also suggest before you do anything, that you make a set of Recovery DVDs using the ERecovery program. You never know if you might need them