Does this only happen with 3D screensavers or all of them? If the former,
XP SP1 has included a fix for this issue.
Other than that, I would suggest going to Start/Run and type in:
Dxdiag/Display and test DirectDraw.
If this is not the case, go into the setup settings for your card and set
antialiasing to No and reboot.
All the Best,
MS-MVP Win98/XP-Shell/User [AE-Windows? XP]
Troubleshooting Windows XP