it wont slow down ur PC.
I have tsted 2 identical machines (100% identical) one with 3.06 GHz 533 Bus Processor and HT Enabled
other with 3.40 MHz (HT Disabled)
that makes the later with higher clock speed and 800 FSB, still it lagged behind the 3.06 that was HT enabled.
and this wasnt tested on just one software, around 7.
alos, it will keep your system cooler,as seldom you'll reach 100% CPU load barrier. as I when on load mine's @ 50% and temperature's @ 39 @ normal.