First of all, make sure you have a good updated antivirus and scan any file you download with your AV before opening it. But the Limewire network is not safe for your PC and my advice is to not use it or any other p2p filesharing networks. Some people deliberately upload viruses on filesharing networks.
Videos4blackberry.info is a safe site with videos already formatted for BlackBerry. Best to download them to your PC then transfer to your BlackBerry.
Other videos can be converted with Desktop Manager/Roxio.
How do I convert video using Roxio - BlackBerryFAQ