Notes :

•  If you decide to updgrade a WSS application to SPS you will have
  to restore each virtual server.
•  If WSS was installed in Active Directory mode or in host header mode,
  it can’t be upgraded to SPS.

To upgrade, follow these steps:

1.  Record the database settings used by each virtual server.
2.  Record the Application Pool settings used by use top-level site in
   IIS.
3.  Install and configure SharePoint Portal Server.
4.  Re-extend each virtual server.
5.  Connect the existing sites to the portal server.
6.  Link the existing sites to the Sites Directory in the Portal.

MSDN complete article : Go to article !