I recommend you to use Avast. There is a free version of these antivirus, and it protects your limewire downloads and lots of other stuff. Before installing this antivirus be shure to completely uninstall norton antivirus. to do so you can go to controll panel, add and remove programs and select nortons products. As this is probably not enough to remove nortons **** products you may also need to go to their site and search for a complete uninstaller for you version or norton.
finally get a good registry cleaner , i recmmend Ccleaner ( it is very secure)
Use also a firewall, I recommend sygate personall firewall as it is also free.
Antispyware recommended tools ( free versions available) : ad-aware, spybot search and destroy ( for advanced users), spyware blaster.
These should be enough.
you can google all the things i mentioned, specially the ones concerning about nortons uninstalling.
i dont recommend having more than one antivirus or firewall at the same time due to incompatibilities.
pd. avast! rocks

Regards