ClearType artifacts

About a year ago, I wrote a basic interoffice communication/notification
app.  Of course they wanted something different, instead of a normal window
they wanted a funky custom shaped one that doesn't steal focus or interupt
work at hand.  Wasn't a biggie, in lieu of a form I just used my custom
image usercontrol (transparency mask, an image for the custom shape window,
etc.) and parent it to the desktop.  Nothing groundbreaking.  Worked fine
all this time until one fellow reported an issue with his new XP notebook
(LCD, ClearType setting).  When he closes the "window" (the UC object does
get destroyed), it leaves a "footprint" or artifact on any text that was
under it.  Any text under it (like text in Outlook) is distorted a bit.  As
if the ClearType "voodoo" isn't getting refreshed or repainded.

Just want to inquire if anyone here knows of such an issue.  Is it something
with ClearType technology or such?