As Thomas said STSADM will make a complete backup of your WSS site or site group into a file you specify then you can back that file up to tape. However if you are using SQL for your backend databases, you can also use the SQL backup to backup the Config and Content databases there. I don't suggest re-usig the config DB, if moving or restoring the sites to them same or a new server.
Also note neither of these backup methods will take into account customizations to your Sharepoint Sites, by that I mean Web Parts, CSS, basically anything that is not a part of the SQL database. If you want to backup those customizations I suggest with NTbackup, backing up the c:/program files/common files/microsoft shared/web server extensions/60/ directory.
If you are using WMSDE, I highly suggest if you can to install the SQL Client tools to administer the WMSDE databases the OSQL command line can be a bit intimidating to use. If you don't have a full blown version of SQL 2000, download the EVAL version from the Micrsoft site to install them.
Later
Jay