Disappearing XP Taskbar

I have a Pentium IV running Windows XP Home which has worked well for many months but recently suffered from worm/spyware attack. I have run numerous scans as per CastleCopsWiki and believe that I have removed the problem – no recurrence of pop ups or slow down for several weeks now.
However when the computer is left idle for a period the taskbar at the bottom of the desktop disappears. I have to do a Ctrl-Alt-Del, log off and log back on to get it back. Does anyone have any idea why this might be happening and how it might be fixed?

Regards
Steve Bliim