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I have to install SL 5 on my pc. The problem is: my pc doesn't have a dvd player and thus I have to make a bootable USB. No problems there too. I can make a bootable USB with one iso file in it. So I mounted the 2 iso's I got from official scientific linux 5 link.

But now what? I can't understand the 'readme'. It says I have to use boot.iso or diskboot.img. But these files are in megabytes only.

Before proceeding please take a moment to understand the Scientific Linux releases and what you can and can't do while upgrading from one to another: https://www.scientificlinux.org/documentation/faq/faq-releases/

SL5 is no longer supported/distributed. Please download and use SL6x or SL7x:

Scientific Linux 6x LiveDVD (64bit)

Scientific Linux 6x LiveDVD (32bit)

Scientific Linux 7x LiveDVD (Gnome) (64bit)

Scientific Linux 7x LiveDVD (KDE) (64bit)

Prefer the 7x release unless you explicitly need 32bit.
Now that yo have the correct ISOs, in order to make a bootable USB media you need to use the dd command or similar or any other graphical tool for the purpose like Rufus for Windows. In Ubuntu MKUSB is strongly recommended:

sudo add-apt-repository universe # only for standard Ubuntu
sudo add-apt-repository ppa:mkusb/ppa # and press Enter
sudo apt-get update
sudo apt-get install mkusb mkusb-nox usb-pack-efi

MKUSB is quite user-friendly. For detailed informations please refer to the quick start manual.

Now that you have a SL live media just boot from it and either run live or install in dual-boot.

If you are uncertain about how to do the installation, and/or you want to install the system only temporarily, and/or you want to be able to configure multiple hosts inside a private network (or whatever other excotic fancy floats your boat), then installing in a virtual machine environment may be a better option than trying to install a dual boot system.

Using, for example, Oracle's VirtualBox, it's easy to set up a virtual machine with the disk size, memory requirements etc. that you need (bounded by the host machine's available resources, of course). You additionally do not need to burn a CD-ROM or DVD as the ISO file may be mounted in the guest OS's virtual CD-ROM drive.

If you mess up, just delete the machine and start over. If you're happy with a setup but plan a big upgrade of it, make a snapshot so that you can get back to a working system if you mess up with whatever you're about to do.

I run three OpenBSD machines on my single Windows laptop, inside VirtualBox. I seldom use Windows for other things than for web browsing and Slack. The virtual environment is where all my work happens.