After you ask for a free license from Bitdefender and they e-mail you a license number, Bitdefender should also send a link in an e-mail for downloading your Bitdefender anti-virus/anti-malware program. Follow that link for Bitdefender for unices. It should be for the Debian version for a workstation (not a server). (If they don't send you the link, e-mail them and ask for it, and be sure to tell them that you want the professional version, NOT the "free" version, and that your use is for home use.)
When you download it, be sure that you download the correct file. The file should be for either the 64 bit version or else for the 32 bit version (according to whether you are running the 64 bit or the 32 bit version of Ubuntu).
If they make a mistake and send you a couple of links for the free version, you may have to contact them again. What you want is a free license for the PROFESSIONAL version for a WORKSTATION.
The file you download should look something like this for the 64 bit version:
BitDefender-Antivirus-Scanner-7.6-4.linux-gcc4x.amd64.deb.run
and for the 32 bit version:
BitDefender-Antivirus-Scanner-7.6-4.linux-gcc4x.i586.deb.run
The exact file name will vary according to the name of the most current version. The link that you follow (from the link in your e-mail from Bitdefender) will probably lead you here.
I suggest downloading it to your Ubuntu desktop. Then copy and paste it into your HOME Folder. To get to the HOME Folder, choose Places menu selection (top of the screen), and then HOME Folder (Places → Home Folder). (Your HOME Folder may have your name or any name you chose when you installed Ubuntu.)
Go to the Terminal (Applications → Accessories → Terminal)
Then type the following: sudo sh NameOfInstallFile
where the NameOfInstallFile is the exact file name of the file that you downloaded.
It will require permission from you (via your password) to do this installation.
For example, for the current 64 bit version, the command line may be:
sudo sh BitDefender-Antivirus-Scanner-7.6-4.linux-gcc4x.amd64.deb.run
or for the 32 bit version, it may be:
sudo sh BitDefender-Antivirus-Scanner-7.6-4.linux-gcc4x.i586.deb.run
As it installs, it will go through the license agreement by percentage, and you will have to press the Enter key repeatedly to cause it to proceed to 100%. As it approaches 100%, you will need to slow down so that you can type "accept" (just the letters, not the quotes) at the exact proper point, or else it will not install. Also, when it asks whether or not you wish to use a graphical-user-interface (GUI), type "y" for "Yes." After it fully installs, it will be available via the Applications--->System Tools menu selection of Ubuntu.
When you use Bitdefender to scan your files, it will by default scan your Home directory. For Ubuntu Linux, that is probably sufficient because that is typically where your e-mail and Firefox files will be, as well as your other information files. However, you can direct it to scan other directories, including even the Windows side of your computer (for dual-boot systems), if you wish. I have noticed that when it updates for the first time, it will sometimes appear to lock up. Don't worry, just wait an additional 15 minutes (to be sure it has updated completely), and then reboot. That problem may not happen again after it first updates.