That usually indicates a firmware update is being applied, but if it happens every time that means something else is causing an issue that it just thinks is a firmware update being applied.
Try starting up the computer by holding down the power button for 5-10 seconds (do this with the computer already off, if you do that while it's on it'll force a shutdown, which can be problematic). You'll hear a long tone and then the same bar will come up. That's the official way to upgrade the firmware (after an update has been downloaded), and it could be one was downloaded and never fully installed properly.
Failing that, try running Disk Utility from your Install disk and repairing the Disk as well as Permissions (Permissions likely aren't at fault here, but it doesn't take that long and doesn't hurt anything to try).
jW