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I think you are referring to the file ATL100.DLL (note the "dot" just before DLL). DLL files are dynamic-link library files, and they are used in the various Microsoft operating systems (like Windows XP, Windows Vista, Windows 7, etc.). DLLs can contain code, data, and resources, in any combination.
Since you're asking about this, I assume you saw an error message that talked about a missing or damaged ATL100.DLL file. This file might, for example, be used with programs like Nero Burning (used to create and edit CDs), or you might see an error message when you are browsing the internet. You might try the methods recommended in this blog [related links].
I suspect you can fix the problem using the first (and safest) method mentioned there (Update Internet Explorer).
Beyond that, find a friend whom you trust and who knows something about computers to "hold your hand" while you try the other solutions. If you don't have such a resource, you're probably going to have find (and pay for) a professional to troubleshoot this problem.
WARNING: The above blog entry talks about making changes to your Registry. If you are a novice, I would not recommend doing that. If you feel that you need to do so, make certain that you do a Registry backup first. When you first run the program regedit, click File, Export. You will be asked to supply a filename for the Registry backup. I suggest a name like Registry_2012-09-11, where the numbers represent the date you do the export. I know people will tell you that you shouldn't save files to your desktop, but Registry backup files are a wise exception to this rule. That will make the backup easier to find if and when you need it.
