CPU easily maxes out in WinXp Pro, IE slow loading

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I really hope there's a fix to this without having to do a reinstall. My dad's computer is running really slow. Internet Explorer and Firefox are both incredibly slow to open (firefox more so). Microsoft office applications appear to open fairly normally. Internet explorer is incredibly slow to load pages. Firefox isn't too bad. Internet explorer maxes out the CPU at 100% for a good while when it loads a page like www.msn.com or www.dailynews.com. Firefox loads up the CPU quite a bit but doesn't quite max it out. On my computer, those pages load very quickly without any problems. His computer is a 1.8HGz Pentium 4 with 1GB RAM. I've deleted temporary internet files and have run Spybot, Adaware, and Norton which didn't really find anything. There have never been any obvious signs of malware on the computer. I've tried disabling all startup items and killing off other running processes. I've tried disabling 3rd party browser extensions. I've tried a new user account. Problem persists. In safe mode everything works a lot better, but we all know that safe mode is not a primary working environment. As far as I know, nothing new was installed recently. I really can't figure out what is causing it. Any help is greatly appreciated. And no "get a mac" or "switch to ubuntu" are not options.