This is what the error log says:
2013-04-27 17:02:14 [iNFO] [ForgeModLoader] Forge Mod Loader version 5.1.8.611 for Minecraft 1.5.1 loading
2013-04-27 17:02:14 [iNFO] [ForgeModLoader] Java is Java HotSpot Client VM, version 1.7.0_21, running on Windows Vista:x86:6.0, installed at C:\Program Files\Java\jre7
2013-04-27 17:02:14 [FINE] [ForgeModLoader] Java classpath at launch is C:\Users\Metaloonie\Downloads\Minecraft (1).exe
2013-04-27 17:02:14 [FINE] [ForgeModLoader] Java library path at launch is C:\Program Files\Java\jre7\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;C:\Program Files\AMD APP\bin\x86;C:\Program Files\Common Files\Microsoft Shared\Windows Live;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\hp\bin\Python;C:\Program Files\Windows Live\Shared;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\ATI Technologies\ATI.ACE\Core-Static;C:\Program Files\Java\jre7\bin;.
2013-04-27 17:02:14 [FINE] [ForgeModLoader] Enabling runtime deobfuscation
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] All core mods are successfully located
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Discovering coremods
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found a candidate coremod GuiAPI-0.15.2-1.4.6.jar
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Loading coremod GuiAPI-0.15.2-1.4.6.jar
2013-04-27 17:02:14 [WARNING] [ForgeModLoader] The coremod sharose.mods.guiapi.GuiAPI does not have a MCVersion annotation, it may cause issues with this version of Minecraft
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Loaded coremod GuiAPI-0.15.2-1.4.6.jar
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found a candidate coremod Tropicraft for MC 1.4.6.jar
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Loading coremod Tropicraft for MC 1.4.6.jar
2013-04-27 17:02:14 [WARNING] [ForgeModLoader] The coremod net.tropicraft.asm.TropicraftCorePlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Loaded coremod Tropicraft for MC 1.4.6.jar
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found library file argo-small-3.2.jar present and correct in lib dir
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found library file guava-14.0-rc3.jar present and correct in lib dir
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found library file asm-all-4.1.jar present and correct in lib dir
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found library file bcprov-jdk15on-148.jar present and correct in lib dir
2013-04-27 17:02:14 [FINEST] [ForgeModLoader] Found library file deobfuscation_data_1.5.1.zip present and correct in lib dir
2013-04-27 17:02:15 [FINEST] [ForgeModLoader] Found library file scala-library.jar present and correct in lib dir
2013-04-27 17:02:15 [sEVERE] [ForgeModLoader] A critical problem occured registering the ASM transformer class cpw.mods.fml.common.asm.transformers.DeobfuscationTransformer
java.lang.ClassNotFoundException: cpw.mods.fml.common.asm.transformers.DeobfuscationTransformer
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:211)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at cpw.mods.fml.relauncher.RelaunchClassLoader.registerTransformer(RelaunchClassLoader.java:96)
at cpw.mods.fml.relauncher.RelaunchLibraryManager.handleLaunch(RelaunchLibraryManager.java:278)
at cpw.mods.fml.relauncher.FMLRelauncher.setupHome(FMLRelauncher.java:172)
at cpw.mods.fml.relauncher.FMLRelauncher.relaunchApplet(FMLRelauncher.java:259)
at cpw.mods.fml.relauncher.FMLRelauncher.appletEntry(FMLRelauncher.java:229)
at net.minecraft.client.MinecraftApplet.init(MinecraftApplet.java:25)
at net.minecraft.Launcher.replace(Launcher.java:136)
at net.minecraft.Launcher$1.run(Launcher.java:79)
Caused by: java.lang.NoClassDefFoundError: net/minecraftforge/event/Event
at net.minecraftforge.transformers.EventTransformer.buildEvents(EventTransformer.java:59)
at net.minecraftforge.transformers.EventTransformer.transform(EventTransformer.java:35)
at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:292)
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:199)
... 10 more
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.event.Event
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:211)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 14 more
Caused by: java.lang.AbstractMethodError
at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:292)
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:199)
... 16 more
2013-04-27 17:02:15 [FINEST] [ForgeModLoader] Running coremod plugins
2013-04-27 17:02:15 [FINEST] [ForgeModLoader] Running coremod plugin FMLCorePlugin
What's going on?