Some software like MRU Blaster by Javacool Software can cause Screensaver problems.
Or try this thread.
XP Pro Screensaver Quit Working
thread779-453976
Windows XP Screensaver Timeout Problem
thread779-639552
The culprit was MRU Blaster by Javacool Software.
If you enable it via Group Policy, a screen saver runs, provided the following two conditions hold: First, a valid screensaver on the client is specified through the "Screensaver executable name" setting or through Control Panel on the client computer. Second, the screensaver timeout is set to a nonzero value through the setting or Control Panel.
The Screen Saver Policy Settings Are Not Enforced on a User's Computer
Check the following Registry Locations
Check the Value "SCRNSAVE.EXE" and "ScreenSaveActive".
See what location it is pointing to, and whether the Access Permissions for the Registry Key are correct.
"ScreenSaveActive" appears in these locations when the Group Policy is not configured. The last two keys may vary depending on users etc.
HKEY_CURRENT_USER\Control Panel\Desktop
HKEY_USERS\.DEFAULT\Control Panel\Desktop
HKEY_USERS\S-1-5-18\Control Panel\Desktop
HKEY_USERS\S-1-5-21-507921405-839522115-725345543-1008\Control Panel\Desktop
If you enable it in Group Policy "ScreenSaveActive" appears in these keys. Again the last 3 keys may be slightly different depending on users.
HKEY_CURRENT_USER\Control Panel\Desktop
HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Control Panel\Desktop
HKEY_USERS\.DEFAULT\Control Panel\Desktop
HKEY_USERS\S-1-5-18\Control Panel\Desktop
HKEY_USERS\S-1-5-21-507921405-839522115-725345543-1008\Control Panel\Desktop
HKEY_USERS\S-1-5-21-507921405-839522115-725345543-1008\Software\Policies\Microsoft\Windows\Control Panel\Desktop
In all cases the valuedata is 1 for active or 0 for disabled (Reg_SZ)