Again, more slowly.
Option 1. Any version of unmodded Xbox, buy a composite to VGA active converter, and it will work but the quality will be variable.
Option 2. Any version of unmodded NTSC Xbox, buy an Xbox component cable and a component to VGA or DVI adapter (like the ones shambles1980 linked to), and if your monitor supports YPbPr over the VGA/DVI ports then it will work.
Option 3. Any version of unmodded NTSC Xbox, buy an Xbox component cable and a component to VGA active converter, and it will work.
Option 4. If your Xbox is 1.0-1.5, mod it (softmod or chip/TSOP flash), enable VGA output, buy/make an Xbox VGA cable (soldering required), and it will work. If your monitor DOESN'T support sync-on-green then you will need to put a sync separator in the cable. See this post
for cable details.
So, if your Xbox is a 1.6 then you can use any of Options 1-3