The message that pops up after limewire is started says,

"LimeWire version 4.18.3
Java version 1.6.0_03 from Sun Microsystems Inc.
Windows Vista v. 6.0 on x86
Free/total memory: 30537280/33357824
java.lang.NoClassDefFoundError: com.limegroup.gnutella.settings.LimeProps
at com.limegroup.gnutella.bugs.LocalClientInfo.<init> (LocalClientInfo.java:61)
at com.limegroup.gnutella.gui.LocalClientInfoFactoryI mpl.createLocalClientInfo(LocalClientInfoFactoryIm pl.java:24)
at com.limegroup.gnutella.bugs.FatalBugManager.handle FatalBug(FatalBugManager.java:59)
at com.limegroup.gnutella.gui.GUILoader.load(GUILoade r.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.limegroup.gnutella.gui.Main.main(Main.java:45)
Caused by: java.lang.NoClassDefFoundError: java/util/concurrent/Executors
at org.limewire.concurrent.ExecutorsHelper.newFixedSi zeThreadPool(ExecutorsHelper.java:119)
at org.limewire.setting.SettingsGroupManager.<init>(S ettingsGroupManager.java:46)
at org.limewire.setting.SettingsGroupManager.<clinit> (SettingsGroupManager.java:24)
at org.limewire.setting.BasicSettingsGroup.<init>(Bas icSettingsGroup.java:33)
at com.limegroup.gnutella.settings.LimeWireSettings.< init>(LimeWireSettings.java:12)
at com.limegroup.gnutella.settings.LimeProps.<init>(L imeProps.java:22)
at com.limegroup.gnutella.settings.LimeProps.<clinit> (LimeProps.java:13)
at com.limegroup.gnutella.gui.Initializer.validateSta rtup(Initializer.java:204)
at com.limegroup.gnutella.gui.Initializer.initialize( Initializer.java:87)
at com.limegroup.gnutella.gui.GUILoader.load(GUILoade r.java:60)
... 5 more
STARTUP ERROR!"

The beginning messge says this,
"One or more necessary files appear to be invalid. This is generally caused by a corrupted installation. Please try downloading and installing LimeWire again. If the problem persists, please visit Lime Wire » Official Website & Free Download, BitTorrent « and click the 'Support' link.
Thank you."

Try LOTR's advice here: http://www.gnutellaforums.com/320995-post377.html