Today I installed the Windows SharePoint Services 3.0 Service Pack 2 (SP2) on my department Intranet prototype. It is configured as a Farm with a SQL 2005 database but everything is hosted on the same virtual Windows 2003 R2 server. Installation instructions for this WSS 3.0 Service Pack 2 (SP2)and documentation are quite smooth and clear.
In short, it all comes to downloading the file, then double-clicking on the exe to run it and then the usual Start, "click", Accept, "click", Next, "click", Next ... wait (Upgrading SharePoint Products and technologies through the Configuration Wizard) ..., Finish, "click", then open your Intranet, wait again, and see the result.
It worked. Simple and efficient.