It really depends on from where you are running this BAT file. If the BAT file is running locally on the server you are monitoring, then you can just add the %COMPUTERNAME% variable to get the system's machine name.
For example, in your BAT file after you generate the text file from Perfmon, you can do something like:
ECHO %COMPUTERNAME% Performance Monitor Alert >machine.txt
That would create a TXT file with the machine name, and the words "Performance Monitor Alert" right after the machine name. This would be saved in the file "machine.txt"
At this point, use your e-mail program to send both the Perfmon alert file AND this machine.txt file.
Then, you will have the alert info and the machine name.
There are other ways of doing this also, but this is a fast way!
hope this helps