Provided it's hosted on a linux box you could do the following:
Assume (for example purposes) your site is hosted in /home/www/mysite and you are in that directory.
Type:
tar -cvf mysite_07212003.tar *
gzip mysite_07212003.tar
then just browse to your site and put the file name in.
http://www.mysite.com/mysite_07212003.tar.gz
...you now have a full backup.
To restore just FTP the file back to the host in the site root dir and issue:
tar -xvf mysite_07212003.tar.gz