Choose About this Mac from the Apple menu and check the processor.
If it's a Core Duo Mac, click here, install the DVD, and run Software Update.
If it's a Core 2 Duo or better Mac, follow the Core Duo instructions, and if desired, go to the online Apple Store and order a download code for Lion 10.7.
Mac OS X 10.7 and newer don't support PowerPC software such as Microsoft Office 2004. If you upgrade the OS, back up the computer first.