Slow taskbar re-appearance after s/saver

After moving the mouse etc. to clear the screensaver, the Windows (XP) taskbar can take between 5 seconds and 60 seconds to re-appear. Until it re-appears, there's a ghosted/black image of the taskbar in stead. I've applied the latest video and NIC drivers. The two affected PCs are of the spec 733MHz/512Mb/10Gb. The Windows XP installation was an upgrade from Windows 2000 rather than a clean install. Any help would be appreciated as the two affected PCs belong to the two co-directors!