Well there is no supported way of doing this. The log file format is proprietary to Microsoft and it is not documented. There is an undocumented command to output some information regarding the log. The syntax is:
In that case, there was an article posted in the last week or so that gave many ideas (as well as its following discussion where even more ideas were presented) on simple ways to audit all changes to data in all of your tables... I would suggest you look at the article - a quick search of the site would find it.
Either that, or simply use one of the 3rd party tools mentioned above. Then you will not need to audit constantly and decrease performance. But you cannot do it with some undocumented command or something, it is as simple as that.