It is normal that AVG shows that files, the MBR or Boot record to have changed. These are done during normal maintainance, when you or windows updates files or have had to correct errors on the drive. The only time that you should worry is if they also show as infected.
To get AVG to quit showing them as changed, open the AVG Test Center, click the F3 key on your keyboard and tell it to accept the changes. If it still shows something as changed after this.. delete the file named AVG7QT.DAT in C:\ and AVG will rebuild it the next time it is run.
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