Runescape unbearable lag

For about a week NIS worked great with no lag (except the first night where everyone had black screen problems). Before NIS, I ran Runescape 2007 Gold on a high graphics setting with audio on. I have a 2003 Dell PC with Windows XP that has never had any problems running Runescape and can run games like TF2 with no problems. Now every time I log into RS, I get this crazy lag. The hitsplats stay above NPCs, it takes forever to move my character and my mouse movement is also slow. I run RS on minimum graphics with audio off. I also tried turning off all other programs and Runescape still has this lag issue. Please resolve it so I can enjoy RS and the Battle of Lumbridge event. I like the new interface, music and world event but I need to be able to run the game!.