secEvent.evt is a Security log file and index.dat is a System file that is the active database of Internet Explorer. These will not defrag and index.dat will grow in time. It keeps a record of your browsing.
If you use CCleaner to rid yourself of cookies and unwanted files which a lot of users here do, it will delete index.dat for you safely and on bootup it is always re produced at it's basic file size.
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