How to install SharePoint v2 Service Packs?

NOTE:
Before you start make sure you have all possible backups Full farm backup, SQL Database backup, stsadm -o backup etc.
Install during off production hours so the files will not be in use.
Install Service Packs in order WSS2.0 SP first SPS03 SP later.

Windows SharePoint Services 2.0 Service Pack 2 (SP2)
http://www.microsoft.com/downloads/details.aspx?FamilyId=9C552114-DC9A-43EE-9281-B080B7C280B4&displaylang=en

SharePoint Portal Server 2003 Service Pack 2
http://www.microsoft.com/downloads/details.aspx?familyid=171DC9A7-B4BA-4759-BD64-6B7D851A97EF&displaylang=en

In case you upgrade to WSS2.0 SP3 & SPS2003 SP3 also apply post sp3 hot fixes

Windows SharePoint Services Service Pack 3 (SP3)
http://www.microsoft.com/downloads/details.aspx?familyid=2BBFC89B-EB59-49FF-B58F-684693CB25A7&displaylang=en

Description of the Windows SharePoint Services 2.0 post-Service Pack 3 hotfix package: January 25, 2008
http://support.microsoft.com/kb/941412

SharePoint Portal Server 2003 Service Pack 3 (SP3)
http://www.microsoft.com/downloads/details.aspx?familyid=AB086D48-1148-48DE-B4EC-A6298367E5F0&displaylang=en

Description of the SharePoint Portal Server 2003 post-Service Pack 3 hotfix package: February 26, 2008
http://support.microsoft.com/kb/943167

In case you face any problem after installing service packs try following commands to upgrade all components & database schemas forcefully.

Open a command prompt
Browse to C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\BIN Run following command:
stsadm –o upgrade –forceupgrade –url [Site URL]