Then I found that I could write my iso files in a usb flash drive and boot from that key. While most of new pc models were capable of booting off a usb drive, some old pc models (on which I usually test ultra-lightweight Linux distrubutions such as "Puppy Linux") cannot boot from usb keys.
However, there is a way to bypass this awful motherboard restriction and boot from usb flash drives without modding the B.I.O.S.(!!!!do not try this method unless you really know what you are doing!!!!).
In this instructable, will bypass this restriction using a linux based bootmanager. This bootmanager can be written on a disk or on a floppy disk.
Step 1: Things you will need
---------->iso burning software (cd burner xp,magic iso......)
Step 2: First burn the boot manager image in a blank cd
The iso is located at the root of the zip file and it is named plpbt.iso
Step 3: Then create a bootable usb drive
Step 4: How to use PLOP bootmanager
While this key is different for every machine, the most common ones are the DELETE, the F2 and the F2.
After you have succesfully entered the BIOS setup, head to the boot section and put the media that the bootmanager is installed in first boot priority.
Finally save your changes and reboot with the media connected to the target pc