I would suggest getting a copy of Working Effectively with Legacy Code.
We went through this book in a study group. it was painful, but useful.
Topics include:
- Understanding the mechanics of software change: adding features, fixing bugs, improving design, optimizing performance
- Getting legacy code into a test harness
- Writing tests that protect you against introducing new problems
- Techniques that can be used with any language or platform-with examples in Java, C++, C, and C#
- Accurately identifying where code changes need to be made
- Coping with legacy systems that aren't object-oriented
- Handling applications that don't seem to have any structure
You can see a short into to this at http://www.objectmentor.com/resources/articles/WorkingEffectivelyWithLegacyCode.pdf
