You are wise in wanting to keep Windows on its own drive.
First, I would say do not uninstall your other distribution or modify your existing drive in any way shape or form.
Buy a new drive and play with gparted to create the partitions on it for the sizes that you are are going to want but if the drive is large enough, leave space at the end of it large enough to later create a partition to back up your Windows drive.
Then, disconnect your windows drive so that there is no chance at all of modifying it unintentionally.
Now, install one or more PCLOS flavors on your new drive and update them.
Now, once you know that the new drive will boot into all of your PCLOS flavors, shut down and reconnect your Windows drive.
If the machine boots into PCLOS, great. If it does not, go into the BIOS and set the machine to boot from the PCLOS drive.
Now boot back into PCLOS and edit your menu.lst file to add a stanza at the end of it that will chain load your Windows drive boot loader.
Reboot and see if selecting that new stanza takes you to the boot loader that you have been used to on your machine up to now.
If it does...and it should... you are are done.
This method allows you to add your new stuff without touching your original drive.
If in the future you have any problems with the distributions on the PCLOS drive, just unplug it and your machine will boot as it does now.
There is almost no risk of losing your windows install this way.
But, personally, I would still back it up first using Acronis or the program of your choice.
Now if you did not already backup your Windows drive, learn how to run one of the disk copying and back up programs available in the repositories and back up your Windows drive to the space you left on the end of your new drive.
Notice that I did not say to use redo MBR. The reason is that there is a good chance that you could inadvertently modify your original drive due to the fact that there are some really ambiguous choices presented to the user when using it. By manually editing the menu.lst file on your new drive only, you do not need to touch the Windows drive.
I would give you the exact text of the new stanza but my memory is such that I would probably get it wrong and you can find it in the forum in multiple places by searching for "Old-Polack" and "chain loading".
Also, the other reason that I say to do it this way by chain loading to your existing drive is that there is a good chance that the distribution coexisting on the Windows drive uses GRUB 2 which PCLOS does not and even though the two don't work well together they are now separate and it won't matter.
Well, it looks like once again I typed too slow and OP got here first.