Back again. This is what I obtained:
First: The problem that I reported with the PCLinuxOS Live USB Creator, using the second partition of the USB Pendrive was solved, once I re-formatted it as ext4. I could make the install of PCLinuxOS LXDE 2012-06 using the Live CD.
Second: I could not successfully boot from the computer using a boot entry from my main installation, which is PCLinuxOS KDE fully updated, with the pae.bfs, kernel 18.104.22.168 for 64-bit.
I have two physical hard drives: one with Windows 7 and the second with PCLinuxOS KDE.
I also have two USB Pendrives: one dedicated for ReadyBoost, and the second one partitioned with Knoppix 7.04 (FAT32 format) its first partition, and the second partition for a Linux Distro (ext4 format), PCLinuxOS LXDE in this case.
The boot stanza that I use for the USB Pendrive is:
title Knoppix 7.04
title Linux Distro USB
Knoppix boots fine. PCLinuxOS LXDE, for Linux Distro USB produces the following otuput:
kernel (hd0,1) /LXDE1/vmlinuz LiveCD=/LXDE1 fromusb root=UUID=50e9131d-4b6e-4f95-b873-33608559fd87 acpi=on vga=791 quiet splash=silent vmalloc=256M
ERROR 17=Cannot mount selected partition
Press any key to continue
This output came in a black screen. I copied this text by hand, and I can recall exactly if all capital letters in my text above were shown as such, but the wording and numbers are correct.
The only way to recover was by a hard reboot, since all entries for the new boot menu of the USB Pendrive partition produced the same output.
I tried two USB installs of PCLinuxOS LXDE with same result. The variant was the grub option from the last window of the facility. I used option 2 for grub install in the partition to allow chainload from my main install, and option 3, for grub install in the boot sector of the partition.
I did a google search for "ERROR 17=Cannot mount selected partition" but I did not find a reference to apply for this case, at least to my knowledge. The closest clue was an indication for a related problem when booting from external USB drive, but could not find how to address this situation. I also read a problem associated to kernel for 64-bit, which is my case. But again I did not catch how to apply or adapt a solution.