Visual Guide to Less Lag

Windows uses a nagle algorithm which controls how data is sent to/from your computer over the internet. It was meant for slower connections to stop congestion. Nowadays, it causes delays in your online gaming.

You can disable it, though. Here's how.
Credit for the how-to goes to Darkmgl of the Mabinogi Nexon forums. I only made it into pictures for the less computer-inclined.

This guide is for Windows XP only! I don't have vista so can't help you there.