If the trackpad has never worked in Vista it's likely a driver related issue make sure you're downloading Vista drivers and/or that they are available. Unfortunately there are still hardware manufacturers who do not have Vista support for all their hardware.
If it has worked in Vista in the past I'd first recommend you disconnect any mouse connected to the laptop (for example, disconnecting any PS/2 or USB mouse) recommend that you verify the trackpad is enable in CMOS. Additional information about getting into CMOS can be found on the below document.
http://www.computerhope.com/issues/ch000192.htmIf the enabled in CMOS because you've already re-installed the drivers unfortunately I'd say that the touchpad is bad. However, it's still a small possibility that it could be some type of driver incompatibility or confliction with Vista. I'd suggest either reinstalling XP onto the computer if you don't believe it is a hardware issue and/or contacting Acer for recommendations on repair.
Additionally if these solutions don't help, you could also do the workaround with just using an external mouse instead of the touchpad.