copy com5 fileName.txt
at the DOS prompt (XP pro) and ever since then my serial monitor's screen with not re-size.
That has nothing to do with the Serial monitor. I suspect this is a coincidence not part of the issue.
the only thing Arduino IDE installs is the com drivers, and I'm not sure I know how to uninstall them
No, the IDE does not install any drivers.
The drivers are already built into the OS. For Windows (I don't know what Windoze is, maybe it is some kind of pirated thing?), you are actually giving it a description file that tells the OS the Arduino is a COM port. They're actually called description files.
Regardless, fiddling with the COM port won't have any effect on the serial monitor's window size. The serial monitor just talks to the serial port. Since the window size is saved in the IDE's preferences.txt file, you can go and modify that.
Exit the IDE, search for "last.serial" and set it to something like:
Save and relaunch the IDE.