The ONLY way it could negatively impact your speed is if you don't have enough RAM and your Windows partition is so huge that OS X barely has enough swap space to run. Otherwise, it's simply a partition of your hard drive, which doesn't really negatively impact performance at all.
Suprisingly, Windows is less demanding of my system compared to Tiger, since I have yet to recieve my 2GB of ram. 512 is just not cutting it for Tiger, and Windows is built for all the crap boxes of the world so it's no suprise that it copes with small amounts of memory.
Granted, I lose basically every feature that I can't live without by using the Windows partition.
Then again, all I use it for is games. Like the windows operating system was meant to be used..... like a toy.