Or perhaps you mean old classes that someone else wrote many years ago. The term "legacy" is used in this context quite often as in "legacy code". This simply refers to code that was written previously and is still in use.
In his book Working Effectively with Legacy Code, Michael Feathers defines legacy code as code without unit tests. Bear in mind that this is his personal definition, but the more people that read his book the wider its usage is likely to spread.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com