1:get the most recent version of java 32 and 64 bit (some things might still need 32bit)
---get rid of ANY old ones---
with multi mc:
2:create new instance with wanted mc version (f.e. 1.4.5)
3:start instance (to make it download original mc.jar)
---close mc---
4:edit instance mods
5:put compatible forge (also f.e. 1.4.5) into "MINECRAFT.JAR" tab
6:start mc for test
---close mc---
7:edit instance mods
8:add compatible ic2 (f.e. ..you guessed it, 1.4.5) into "MODS FOLDER" tab ...NOT core mods
9:start instance
done