When this sort of thing happens, there's usually an IRQ conflict involved with your soundcard. Oftentimes, what happens is the PCI slot of your soundcard will be competing with your graphics card, or some bs like that.
Here's a good link for tweaking Windows for audio purposes, in general, and it goes in to good detail about IRQ problems:
http://www.pcmus.com/TweakXP.htm
After resolving any IRQ issues, then it's a good idea to get in to some of the common tweaks for Windows (optimising for background services, disable automatic updates, etc). Also, the way Audition works is kind of different from other programs, so if your hard drive is more than 1/2 full, or it hasn't been defragmented lately, then it can skip and sputter during playback. It's kind of funny like that.
Also, if you're using two hard drives ... first off, it's a good idea to make sure that your primary Temp folder isn't on the same hard drive that the program is installed on. Don't ask me why, but for some reason it can make a difference. In general, it's a good idea not to have any programs at all installed on the hard drive that is housing your audio tracks. No plugins, no programs, etc. The ideal is to have the program, all your plugins and everything else installed on to your C drive. While your secondary hard drive should contain nothing but audio files, should always have at least 40% free space or better, and should be defragmented regularly.
