An odd, frustrating but very easy to fix problem appeared on a old Windows 2000 Professional system recently.
The check boxes, task menu arrows and window minimise, maximise and close graphics all changed and made some aspects of the OS more difficult than usual to use. The user thought a virus or spyware had done this but after realising that these parts of the Windows user interface are characters from a TrueType symbol font, Martlett, the solution is trivial. As in print when a font is missing, software substitutes it with another one that usually always looks odd.
The font was installed but corrupt, using a copy of Martlett from another install fixed the problem immediately. MS has something to say about this here: http://support.microsoft.com/kb/134861