Opposite problem
After updating to build 5160 my taskbar which was always visible in 'windowed' mode suddenly disappeared. It would appear briefly after windows boots, then quickly disappear and I couldn't figure out how to get it back.
Following this thread I found the show/hide function (also available by right-clicking on the parallels icon in the dock and then selection application -> show/hide taskbar).
Once I re-exposed the taskbar, it then was stuck at the top of the window (normally always at the bottom) and I couldn't move it. I had to 'unlock' it, move it, 're-lock' it and make sure it was set to display.
VERY annoying that the update caused this sudden change in behavior.