The exclamation mark means that iTunes can't actually find the song file - you've got it somewhere other than the default location, which is usually program files -> iTunes Music, or similar. Make sure you actually put song files in that folder; and I suggest you create a folder for each artist and put album folders in that. That means you end up with a file structure of, for example, iTunes Music ->Fleetwood Mac ->Rumours - where the Rumours folder contains the mp3s of that album. It's also a good idea to just whizz through the tags on each track to keep them tidy.
Incidentally, if you must use Limewire, be very careful to make sure that your PC is properly protected and that you do regular scans with all your AV, anti-trojan and antispyware - Limewire is a notorious vector for getting crap on your machine. And also be aware that downloading tunes via Limewire is theft.
There are third-party applications you can use for an iPod. I like Xplay, which is paid-for and still I think not available for Vista.