Go into b1.7.3-modded and replace all file's name with the new folder name.minecraft/versions, copy b1.7.3 and name it b1.7.3-modded (or something similar) Start the game with the new profile and close it again (just so it downloads the jars).Open the Launcher and create a new Profile with the Beta 1.7.3 version.Installing (Using the 'New'/Second-Oldest Launcher, Not the Very New One!) ModLoader Fixed Version b1.7.3 v1.0.0 ( Direct).Please let me know should any link be broken!
It's a complete version of Modloader, you don't need to install the broken Modloader first, only the fixed one.
If you just want to use the fixed version of Modloader you only have to download the 'Modloader Fix'. Where you can find out anything for utilities of Minecraft.A fixed version of Risugami's ModLoader for Minecraft Beta 1.7.3 that fixes the "URI is not hierarchical" exception.