The info in MilesAhead's link appears correct. Basically, you can create .reg files to hide or show the XP system tray. To hide, enter into a reg file:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"NoTrayItemsDisplay"=dword:00000001
To show, enter into a .reg file:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"NoTrayItemsDisplay"=dword:00000000
Also, you may have to restart explorer.exe for the modification to take effect. I have confirmed on my own XP that this works.