This is the second post I have written on this topic, the first post was Convert Static HTML Site to WordPress Easily. In that first post I covered a very simple way to convert a static HTML site to WordPress.
After almost 2 years of answering questions on that post I thought it was time to discuss the issue again.
I have done a dozen or so conversions since that post and I have found that some sites really need to take a fresh start approach to get the best result. The main reason for this is that a lot of older HTML sites are built with outdated techniques and WordPress has made some great advancements.
On these sites I create a sub folder for the WordPress install, such as /wp. This gives us designated area to work and learn WordPress without interfering with the original design. The hardest task may be getting a theme to match your original design, but I would suggest taking this time to update your original design. There are so many good WordPress Themes this should not be a problem. Once your theme is in place you can copy and paste your original content to the new install matching the page names. Depending on your original file extensions you may need to set the permalinks in WordPress.
Once this development is complete it is easy to change the Site address (URL) in WordPress to show the new site in the root. (Make sure to backup the original first!) Here is the link from the WordPress Admin panel that explains this.
If you need to learn WordPress and want to take a little time without being pressured to get the new site in place this is the best option I have found.