The Office 2003 Web Parts can still be installed on WSS 3.0 and MOSS 2007.
STSADM.EXE -o addwppack -filename "Microsoft.Office.DataParts.cab" -globalinstall
For your clients to see the Web Parts correctly rendered they will need the Office 2003 client libraries and Internet Explorer 5.0 or above.
To use either the Spreadsheet Web Part or the PivotView Web Part, you must have Microsoft Internet Explorer 5.01 Service Pack 2 (SP2) or later and the Microsoft Office 2003 Web Components installed on your computer. See the Microsoft Office Web site for more information.
If a computer has Office 2007 Client or no MS Office at all installed, they will need to download and install the client libraries (OWC11) manually.