Hi, if it's not already installed you need to install the latest touchpad driver, available from :
http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R120179&SystemID=INSPIRONI6400/E1505&servicetag=&os=WW1&osl=en&deviceid=283&devlib=0&typecnt=0&vercnt=2&catid=-1&impid=-1&formatcnt=1&libid=17&fileid=160150Then you should have a synaptics or dell touchpad icon down on the bottom right of the screen near the clock (system tray).
Double click on the touchpad icon to adjust the scrolling settings .
Regards
David