Its pretty simple indeed.
1) Download the XP Pro SP3 (or 2) ISO
2) Burn the ISO to a CD at the lowest possible burning speed (reduces chance of data corruption etc)
3) Take a backup of the stuff you need
4) Reboot the PC with the XP CD in your drive
5) PC should boot from the CD. Follow the installation instructions
Via the installer you will be able to quick-format your harddrive. Format the Vista partition and just follow the rest of the instruction. It's pretty straight forward.
If your PC doesn't boot the XP CD you will most likely need to change the boot device priority in your BIOS setting. I'm not gonna get into that detail because BIOS's can differ a lot. Especially with laptops.
Edit:
Whoops, didn't read the OP well enough. Acer doesn't make its own Windows, they just alter a few things. Installing XP on the laptop should be no problem at all and as simple as installing it on a no-brand PC.