that what you admit here. Pendrivelinux.com is a great site. They wouldn't have infected installation files. There's also "integrity check" on each DVD or flashdrive created which can be run to insure the md5sum for each main folder on the system matches what the original issue ISO has. The way that site works is this;
You install the Universal USB Installation program from there.
You then run the program and you have a choice of Linux systems.
You choose one.
You OK for it to install the particular distro onto the flashdrive
At this point is it possible you chose the hard drive instead of the flashdrive? I don't even know if it would then install the program to the hard drive since that's SATA or PATA system and it's supposed to be for use on USB.
At what point in this episode did you feel or determine you only had Ubuntu available to run on the computer. Can you boot fully into Ubuntu now? If so, then open the Konsole or Terminal or XTerminal program and copy the following commands and post results. The "l" is small L, not numeral one in below commands. Linux is case sensitive on commands.
inxi -v5
sudo blkid
df
sudo parted -l
Linux Mint is my main operating system and I'm quite familiar with the Universal USB Installer program from that site. The above commands will let us determine the system, and the drive setup, the partitions and sizes.
Nothing you've said above could have caused what you seem to describe unless you'd chosen "Install" instead of "Try" on a choice screen, and even then you'd have had a second screen to choose the method of installation, tipping you off a need to clk on the "Back" button (or maybe it's Cancel button).