via the vendor site only. XP doesn't always know what a newer driver looks like. XP will save the older driver (usually) so you can roll back if necessary.
I did a scan at driveragent.com and it recommended upgrading a number of drivers. I went to device manager and tried to upgrade drivers by allowing Windows to search for new drivers at Windows Update. In each case, it reported a better driver was not available. Should I go about this a different way, or just forget about it? I run Win XP Pro 5.1 on an EMachines T6522 with AMD 64 3500+ MMX 3dNow with onboard MSI Radeon Express 200 video and Realtek AC97 sound. I have no current problems, but have read where one should periodically upgrade drivers.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic