Is rebooting the computer normal before/during flights?
Today's modern airplanes have many computers, and sometimes things get out of sync and don't communicate properly.  During the preflight checks, we check each system according to the recommendations of the manufacturer, and occasionally one of the checks will not pass correctly.  Depending on the system, when a check fails, rebooting the computer may be an option to see if it fixes it (similar to rebooting your home computer).  Other times, the issue is the communication protocol between two (or more) different computers.  Often times in this case, the entire airplane must be powered down in order to reset them all at the same time.  Since these tests aren't performed in the air, the problem usually doesn't crop up there.

In the vast majority of cases it isn't a safety critical item that we are dealing with (where the software testing is more thorough), but rather a monitoring system.  The safety critical items (like flight control software, etc.) will have multiple redundant systems and often have different versions of software on at least one of the computers "just in case".

Generally the flight crew won't tell the passengers when they need to "reboot" a computer unless we have to power down the entire airplane.  That's very noticeable and will cause some people to worry so we like to give you a heads up before all of the lights go out and the air conditioning system shuts down.  That of course is up to the discretion of the flight crew though.