In Windows XP, the notification area keeps icons for programs you may long have uninstalled, but even if you go into the Taskbar options, there's no way to clear these 'Past Items' from the list. Here's a quick and easy way to do it.
How to delete past items in taskbar
Click the Start button, then Run… and type regedit in the box which appears.
The registry editor will open. You need to be careful what you do as deleting the wrong things can stop Windows from working. But if you follow these instructions exactly, there's nothing to worry about.
Using the tree structure on the left, navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify
On the right-hand side you will see two entries in the list: IconStreams and PastIconsStream. Right-click on each and choose Delete. Accept the warning each time.
Close the registry editor and right-click on the taskbar. Choose Task Manager.
Click on the processes tab (if it isn't selected already) and then click on Image Name to sort them alphabetically. Scroll down until you fine explorer.exe. Now click on the End Task button or right-click and select End Process.
Click File, then New Task (Run…) and type explorer in the box. Explorer will restart and the past items list will be empty.
You can check it by right-clicking on the Taskbar, choosing Properties and then clicking the Customize… button.