LOL....that is what it will say about any single cpu tasked app that runs with HT enabled...trust me it is using what would be 100% if you turn HT off.....Just go into Bios and turn it off....it will not screw up winxp at all...I did hundreds of times in my testing I did about 1-1/2 years ago....
With winxp I have never seen an app do better with it off when running 1 app.....multitasking sometimes HT would slow down things and not better prioritize as I would have liked it to, but it was still very smooth even when gamiong. It seemed to know to give my gaming (what little of it) about 90-95% power at all times...BUt that was while doing things like encoding and Folding....
I dont think you will see really anything of an improvement...