there are loads of things it could be, but why are you using Kazaa thought that was oldskool? Bitlord is better surely? like RSnissan says it could be memory so check your task manager, but also delete all your cookies and temp internet files cos the slow down pcs no end, and if there are files linked with Kazaa then that might slow down the pc. plus the main folder for Kazaa that store oll the downloaded files will take longer to open the more is in it, so organise those files aswell, as kazaa will use it in the background that might slow you down.
oh and finally get a good anti-virus but not norton, lol.
hope that helps