I`m assuming you`ve checked everythings as it should be under the display properties/screensaver tab.
Quote from here -
http://www.kellys-korner-xp.com/xp_s.htm#ss
"If so,to check your settings : Go to Start/Run/Regedit
HKEY_CURRENT_USER\Control Panel\Desktop. Scrnsave.exe = C:\Windows\System32\logon.scr
Verify your Time while there as well and choose accordingly: ScreenSaveTimeOut = (set your preference)
Also make sure none of the screensavers are wrapped in double quotes as in: ""c:\windows\system32\screensavername.scr""
And try adding: ss in front of the screensaver name.
Screen Saver Disable and Screen Saver Enable (Line 51)
http://www.kellys-korner-xp.com/xp_tweaks.htm "
Other known issues include Wireless mice and Power DVD software.