Your laptop uses an Alps driver (as do most Dells). Dell has their own version(s) of the driver which incorporates scrolling. You can find the latest driver using this search
. Dell now rebrands its Alps driver as a Dell Touchpad / Pointing Stick (then version number). The current version was released on 2/23/2009 and can be downloaded here
Since you're using a much older version of the driver, I would advise you to first uninstall your old driver before installing this new one so there will be no conflicts. Once installed you'll find a number of new features (including scrolling) that wasn't available with the older driver you're currently using.
I hope this helps