ok so i just installed the forge and IC2 1.81 for 1.2.3 and i got this

Mods loaded: 5
ModLoader 1.2.3
mod_MinecraftForge 1.4.0.44
mod_ModLoaderMp 1.2.3v2
mod_TooManyItems 1.2.2/1.2.3 2012-03-03
mod_IC2 v1.81

Minecraft has crashed!
----------------------

Minecraft has stopped running because it encountered a problem.

--- BEGIN ERROR REPORT 72f61fb7 --------
Generated 3/19/12 9:01 PM

Minecraft: Minecraft 1.2.3

java.lang.NoClassDefFoundError: forge/IEntityInteractHandler
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at mod_IC2.load(mod_IC2.java:315)
at ModLoader.init(ModLoader.java:890)
at ModLoader.addAllRenderers(ModLoader.java:186)
at aho.(aho.java:79)
at aho.(aho.java:9)
at net.minecraft.client.Minecraft.a(Minecraft.java:395)
at net.minecraft.client.Minecraft.run(Minecraft.java:732)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: forge.IEntityInteractHandler
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 19 more
--- END ERROR REPORT 63998821 ----------