REG: Missing System Tray Icons in Windows XP

If you google the topic you will find this link:
http://www.techzonez.com/forums/archive/index.php/t-16911.html

It looks all professional, but it isn't actually helpful.

The cause of the problem for me was when I was setting automatic login on my laptop to true (with a password), then later going in and disabling the automatic login.

I did however find a solution.

In your registry editor, search for keys named "NoTrayItemsDisplay", and delete them.
ie:

HKEY_LOCAL_MACHINE
\SOFTWARE
\Microsoft
\Windows
\CurrentVersion
\Policies
\Explorer
"NoTrayItemsDisplay"=dword:00000000

or 

HKEY_CURRENT_USER
\SOFTWARE
\Microsoft
\Windows
\CurrentVersion
\Policies
\Explorer
"NoTrayItemsDisplay"=binary:00000000

source: http://www.techzonez.com/forums/archive/index.php/t-16911.html