From the Microsoft knowledge base:

To take ownership of a folder, follow these steps:
1. Right-click the folder that you want to take ownership of, and then click Properties.
2. Click the Security tab, and then click OK on the Security message (if one appears).
3. Click Advanced, and then click the Owner tab.
4. In the Name list, click your user name, or click Administrator if you are logged in as Administrator, or click the Administrators group. If you want to take ownership of the contents of that folder, select the Replace owner on subcontainers and objects check box.