Explorer.exe uses 50% CPU, and Windows Explorer won't launch

1) First of all try the System File Checker:
  Click Start
  Click Run
  In the Open box that appears, type->
  SFC /PurgeCache <ENTER>
  SFC /ScanNow <ENTER>
  You may need your installation source (CD) as this process will replace corrupted drivers on your system.
  http://support.microsoft.com/?kbid=306084
  http://support.microsoft.com/?kbid=304449


2) Try removing items from Startup:
  Click Start
  Click Run
  In the Open box that appears, type->
  MSCONFIG <ENTER>
  In the Startup tab, start out by disabling everything you're unfamiliar with (or everything if you're
  unsure).
  Optionally, you can also disable non-Microsoft services from the Services tab.
  If the problem no longer exists after a reboot, then you can narrow it down as one of the items in your
  startup.  To permanently remove these item(s), proceed as follows...

  Click Start
  Click Run
  In the Open box that appears, type->
  REGEDIT <ENTER>
  *Be careful when editing the registry as an accidental deletion can render your system inoperable.
  First navigate to the following key in the registry:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
  Once found, click File, Export to save a copy of the key before you delete any items (if necessary).
  After the file has been saved, delete items as needed from the right pane.
  Now find the next startup key:
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
  Follow the previous procedures to export a copy before deleting items from the right pane.

3) Always remove temp files from your system on a periodic basis:
  Click Start
  Click Run
  In the Open box that appears, type->
  %TEMP% <ENTER>
  This is your profile's temporary folder location.  All files can be deleted here, but not the containing
  folder.  Some files may be in use, so an error may be generated but can be ignored.
  The system Temp folder is typically located here: (Start->Run->%systemroot%\temp <enter> )

4) Try stopping/disabling unneeded services from stealing your resources.  http://www.blackviper.com/WinXP/servicecfg.htm

5) Norton's autoprotect has been known to cause high cpu utilization.  Try re-registering autoprotect: (http://service1.symantec.com/SUPPORT/nav.nsf/b69c799adfa31ecc85256aa30052f4d0/f0c69fcb50e2eeaa85256b180068dbf5?OpenDocument&src=bar_sch_nam)

 To unregister and then register the Auto-Protect service:
 Exit all open programs.
 Click Start, and then click Run. The Run dialog box appears.
 Type the following and press Enter:
 "C:\Program Files\Norton AntiVirus\navapsvc.exe" /unregserver
 NOTE: This is the default location for NAV installed as a stand-alone product. If NAV is part of Norton SystemWorks (NSW), then substitute the program path  for the NAV module of NSW.
 Click Start, and then click Run. The Run dialog box appears.
 Type the following, and then press Enter:
 "c:\Program Files\Norton AntiVirus\navapsvc.exe" /service
 NOTE: This is the default location for NAV installed as a stand-alone product. If NAV is part of Norton SystemWorks (NSW), then substitute the program path  for the NAV module of NSW.
 Close the Services program.
 Open NAV.
 Click Options, and then click Auto-Protect.
 In the right screen, click "Enable Auto-Protect (Recommended)" and "Start Auto-Protect when Windows starts up," and then click OK.
 Restart the computer.
 
6) If it still doesn't work - try booting XP clean:  http://tinyurl.com/2ntge (continuation of MSConfig that includes limiting all non-MS services from loading on boot up).

Good luck!