A very common question among users of Drupal is, "How do I upgrade to the latest version of Drupal?" Drupal releases often contain instructions for how to upgrade your site from a previous release of Drupal (for example, 4.6 to 4.7), but quite a bit of confusion remains for updrading from much older releases (such as 4.3 to 4.7). The folks at 2bits offer some advice on how best to update your Drupal site in their post, Upgrading From Old pre-4.6 Drupal Releases To 4.7. The post is short yet helpful in getting you started on a Drupal update.
After looking at the 2bits article, you'll see they suggest the use of a CVS client to install any files changed from older version to 4.5. If you're like me, your initial reaction on the suggestion of using CVS clients and applying patches is the emotion of fear. However I'm quickly finding out that there isn't really much to it. Applying and creating patches is a lot easier than you think.
While there are a number of how-to manuals out there for using CVS and patches, I found the handbooks at Drupal's Website to probably be the most informative. Even if you have zero interest in using Drupal, I think you'll find their manual on applying and creating patches informative and useful for any software/script development you later plan to share or collaborate with others.