I don't have Win 7 but...

Which version of FW & Java are you using (Help > About FW will show you)?

Try updating your Java (or reinstalling, if you already have the latest update): Java Downloads for All Operating Systems - Sun Microsystems
Download the JRE offline installer & remove any other Java versions (via Control Panel) before installing this one. Restart your computer after installing the new Java.

If that doesn't fix things, delete FW's preferences folder when FW's properly closed. See point 3 here (refers to LimeWire but things are the same for FW):Fixes for LimeWire
This will change all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through FW's tools > options & write them all down, so that you can reset things later.
I'm not sure whether you'll need this info for Win 7 as well (it may be the same as for Vista). Again, refers to LW but should all be the same for FW: LimeWire Pref Folder in Vista

Let us know if that helps at all.