xp runs s l o w

I’m running XP Home, no SP2. I have a broadband (cable) internet connection. I was running Notrton the other night, did a full system scan and left the PC up all night. Now XP runs extermely slow opening apps, and tries to connect to the internet but nothing happens. Processor is running at around 100%. I can get to msn.com after about 10 min. Was I hijacked? Any ideas for restoring performance? Thanks!