I just wanted to add what I experienced with this error as well.

I kept getting this error over and over and could not figure out why. I could not connect to any server from the start. I would always get the 10053 error.

After not finding any solutions, I reinstalled XP. With nothing else on my PC, except for mIRC, the error disappeared. YAY!

I then proceeded to reinstall all of my programs. After installing McAfee 8.0, mIRC stopped working again, and I was back to the 10053 error. I then uninstalled McAfee 8.0, and Bam!, the error disappeared. I am reverting back to McAfee 7.0 because I never had the 10053 error when using that version.

Just wanted to add this to the list of possible problems. There could be possible conflicts with you AV software.