Taskbar items flicker

This is the second pc running windows xp professional where any minimized
windows in the task bar flicker and can't be maximized. Also, the expansion
arrow for system tray icons continuosly jumps left and right or up and down
depending on horizontal or verticle placement of the task bar. Only thing
that seems to fix this is rebooting. Any ideas on the cause? One was a brand
new pc and the other was an older tablet pc. Both computers were clean of
malware. Any ideas on the cause?