I will try to walk you through this step by step although systems are different.
Once you have downloaded the zip file, unzip to a folder of your choice. (Make sure you remember the folder name you will need it in a bit)
Now Find the My computer icon (usually you have to left click on the Start button on an XP machine but sometimes it is present on your desktop.
Right click on the My computer icon.
Select Manage (Left click)
Select Device Manager (Left click)
Click the + next to the Display Adapters on the right side of the
Computer Management Window to expand the tree.
Right Click on the Device driver listed just below the display adapters
in the tree. (It should say something like SiS650.
Left click on Properties
Left click on the Driver Tab
Left click on the Update Driver Tab
Select Next
Select Search for a Suitable Driver and click Next
Unselect everything except specify a location. You want to force it
to select your driver from the folder where you just downloaded your
updates. Type in the folder name from above. (You wrote it down hopefully or
you can browse for it.)
Click Ok and it should perform the update and then ask you to reboot.
After you have rebooted, if your display does not look quite right, you may have to set the properties of the screen. Just right click on a blank area on
the screen select properties and set them to where you are comfortable.
After you have done the above, click start. Click run and type in dxdiag.
Click on the display tab and see if the driver was updated. You can then run diagnostics by clicking on Test Direct3d and Test Direct Draw. (You will see spinning cubes for 3d and black and white squares for direct draw if things are going well.
Note : you probably should run dxdiag beforehand and record the driver and file information. If you have to you can go through the same steps above and revert to the old driver. XP is pretty smart about things like that.
I hope I have provided you with enough details to work through this. Again don't get discouraged, I have a degree in Computer Science and it still took me some work to get up and running (Mainly because I didn't read the read me files that the driver manufacturers always include in their downloads)
Good Luck