Besides the 'BAGMRU' option (which did repair my XP a while ago) I found yet another MS Article:http://support.microsoft.com/?kbid=912650&SD=tech
Unfortunately the outcome of this one is: "This is a known issue in WinXP"....
Tooltips for some taskbar icons are hidden behind the taskbar on a Windows XP-based computer
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Article ID : 912650
Last Review : January 20, 2006
Revision : 2.1
When you move the mouse pointer over the taskbar icons on a Microsoft Windows XP-based computer, some tooltips are hidden behind the taskbar.
This issue may occur if you right-click and then open a file from a submenu in the Start menu. For example, this issue may occur when you click Start, point to My Documents or My Recent Documents, right-click a document, and then click Open.
To resolve this issue, use one of the following methods:
• Log off and then log back on to the current account.
• Restart your computer.
This is a known issue in Windows XP.
Will keep on the lookout for you...