If you re-installed Windows XP from an original XP disc, it doesn't contain all the drivers for your machine's hardware.. Visit the computer manufacturer's website and download, then install all necessary drivers for your computer model..

Of course, you don't mention whether you've enabled the "standby button" in your "power" settings.. To be sure, it's enabled RIGHT click on a blank space on the desktop, choose "Properties", then click on the "Screen Saver" tab, then the "Power" button. Once there, click on the "Advanced" tab, then make sure "When I press the Sleep button on my computer", is set to "Stand by".

Hope this helps.