Hello LeeB...welcome to Malware Removal!
I am glad your system is doing well. I found this link at the AVG forum that describes what you are referring to:
http://forum.grisoft.cz/freeforum/read. ... #msg-97105
Seems that it's not unusual for AVG to note 'changes' to the MBR/boot record, and according to the moderators, you shouldn't be concerned unless AVG shows them to be 'infected' vice 'changed'.
The link also shows you how to fix the issue with AVG reporting these files as 'changed'.
If this doesn't answer your question, let me know...or, if you are having any problems with your computer let me know and we will go from there.