When our account was updated (changed from contacts to people) changes were made within our groups that we did not make. How can this be avoided in the future?

Someone we contacted on the internet asked to access our computer.  They found some corrupted files and told us that for $150 they would correct our problem.  That was not in our budget.  Is there a way that we can correct this problem and get rid of possible corrupted files without all the expense.  Your help is appreciated.