A boot record is a small piece on a media that is read by the hardware's firmware in order to load software into the memory (ram) of that system.
This software can be a operating system, but doesn't have to be.
Since you only see it on one system and not on others while they all have the same avast version installed, the problem is not with avast but with the system.
It can be the OS, drivers, hardware etc.
Since the system is running XP, I assume the drive is a old one.
A good start would be cleaning the lens.
Something else you can do is swapping the drives of two systems and see if anything changes.