The only way to do this is to uninstall Windows 7 and install XP. However, before doing so, you'll want to make sure that there are XP drivers for your Aspire notebook. If it's a newer laptop, there is a good chance that it will not run XP (as it's an old Operating System). This explains how to uninstall Windows 7 - http://support.microsoft.com/kb/971762
If you want to install Win XP, then it will mean you have to wipe out your Win 7 installation and that means it will delete all your files and settings so make sure you back what you want before you install Win XP over the top of it.
Just a thought, did you laptop come with Recovery Discs with Win XP on it? If it does, then you can just use that and don't have to worry about the drivers. Rememberr to backup what you want fro your current Win 7 setup as it will delete everything when you install Win XP - unless you want to dual boot it (not sure why you would because Win 7 has Win XP Mode anyway).