If you want to restore the back up, first create one top level site and remember its URL :) say http://test1.com
Then take the back up of existing site using commnd prompt and take the bin directory using command. I installed sharepoint server in C drive
cd C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN
Now we are inside bin folder
then run the command to take the backup
stsadm -o backup -url http://test2.com -filename "D:\
then a message will come "Operation Succesful"
Now we need to restore the backed up site "test2.com" in http://test1.com
Simple, don't leave from command prompt and make sure you are inside bin directory
then run the command to restore back up
stsadm -o restore -url http://test1.com -filename "D:\
then a message will come "Operation Succesful"
Enjoy,
Geogy