With so many dll files being used all the time during your computer’s normal function, damage to said files is not uncommon. While this can happen as a result of a variety of causes, operating system bugs, hardware failings, etc. , the more common reason is the negative influence of malware infections from online files. Ironically these malware may get on your system during the repair of the same files they affect by downloading from bad websites.
This damage is easily avoided by installing anti-virus software and using it to scan all downloaded files (especially dll files). Also, you should perform routine system scans daily, if possible or, if not, weekly. Such simple measures would prevent more than half of computer viral infections that cause damage to you system files.