Now that you have Perl installed, you can do the following:
Create a folder to work in. Copy and paste all of the files you want to edit into this folder, making sure nothing else is there. Now then, use Notepad to create a new file with the following text:
perl -i.bak -p -e 's/<p.*?>/\n/g; s/<\/p.*?>//g;' *.txt
And save it in that folder as htmltotxt.bat. The *.txt means edit all files that have a .txt extension in the current folder.
Then double click it. The folder should now be filled with .bak backups of the original files.
The .txt files should now be stripped of the <p> and </p> tags. I'll explain what the 's/<p.*?>/\n/g; s/<\/p.*?>//g;' means later. It's what's called a regular expression (or regex).
You can also use:
s/<br.*?>/\n/g;
To replace <br>, <br /> and <br anything> with linebreaks.
So in the end, you'd want to use a .bat file with:
perl -i.bak -p -e 's/<p.*?>/\n/g; s/<\/p.*?>//g; s/<br.*?>/\n/g;' *.txt
EDIT: I'll also teach you how to write your own Perl scripts (.pl) later, instead of using .bat files that call Perl from the commandline. This will allow you to do a lot more.