If you got a legal copy for AU$25 rather than an Anytime Upgrade then yes it is well worth the extra work you will need to do; a custom install is required.
Do you have an external harddrive to back up your current data?
It is recommended you backup all your drivers. Some information and a choice of two products that can help are listed
HERE. Easiest would be to burn these to a CD.
Gather all of your programs install disks because they must all be reinstalled fresh after the new OS is in.
Type
Windows Easy Transfer into the start search and use it to back up you accounts and your data. Be sure to check the advanced options to verify it has all your data selected. Back up all this to an external harddrive and shut down and disconnect this data.
Boot up to the Ultimate DVD and you need to tell it to custom install & format the drive and install. There is a tutorial
HERE that should help (print it out or write down any important points prior to starting the procedure)
After the system is installed boot up and install any missing drivers.
Run WET to restore your accounts & data.
Install all your programs.