At risk of repeating myself...
The correct way to install stuff on Linux is to use the Package Manager, Synaptic, for Mint.
Find Synaptic Package Manager and open it, you will be prompted for your password. In the search box type Anti virus and search. Clamav should be listed, tick the box and "Mark for installation" Then Apply.
To update your system. Open Synaptic, select Reload, Mark all updates, Apply. Once completed Logout and log in again.
Hope this helps, come back if you have any more questions."
During the installation you would have been given the optionss
"Use entire hard drive" or
I have paraphrased the exact wording but it would appear you selected the first option. If you want to dual boot then you will need to install Windows first, deleting the Linux install and then re installing it later. Windows is less tolerant of other operating systems on the same PC than Linux.