If you go here
with Internet Explorer and scroll down near the bottom of the page, it looks like you need the Realtek Audio Driver and Modem driver (see edit at end of post).
Before you do, you should open Device Manager (click start, right-click My Computer, click Properties in the menu. Then click Hardware tab and click Device Manager). Check Device Manager for any other missing drivers (look for yellow question mark with an exclamation point next to any devices). Anything else you need should be on that page I linked to. If you cannot access the internet with the notebook without the drivers, then download the files to another computer and burn them to a CD or put them on a USB stick to transfer them to the Acer laptop.
Once you are able to connect to the internet with the laptop, you should go to Windows Update and check for updates and confirm the copy of XP the previous owner installed is legal. Windows Update may also supply the drivers you need.
Hope this helps,
edit: the link didn't quite work, select notebook and Aspire 3100 from the drop down menus to get the list I refer to above
Edited by shinybeast, 23 January 2009 - 01:10 PM.