There are many situations that would present a need for getting rid of the dropdown navigations in Magento - no worries the subnavigations will still show up on the left sidebar. That being said - this is how it is done:

1. Log in to Magento Admin
2. Hover over system and click configuration
3. On the left navigation click on catalog
4. Scroll down to "category top navigation"
5. For maximal depth type in 1
6. Click Save Config

That should do it - leave any questions in the comments section below.