First, if mIRC gobbles resources like you say, something is seriously wrong with your system. Your best approach is to fix this problem rather than ignore it. I am running 3 copies of mIRC 24/7, and occasionally a 4th (yeah lame, even though mIRC does multiserver stuff now I still prefer one mIRC for each network I am on, plus a bot on its own copy), and I have never experienced problems with the use of resources that were directly related to mIRC. I have done so on windows98, 2k Pro and XP Home.
That said, you either run mIRC or you don't. If you want to run a mIRC script, you have to run mIRC.
There are alternatives of course, such as windrop or eggdrop. Dunno much about windrop, eggdrop requires you to rent a shell account (or run it off a unix/linux box of your own), but mIRC scripts will work on neither, you need a TCL script for those bots.