Prefs -> Modern Skins -> General tab
Move "Timers Resoltion" slider to the right (eg. try 70ms, then 100ms, etc)
You didn't say what your Windows OS is, but if it's Win2k or XP, then try unchecking "desktop alpha blending" (Alpha Blending tab)
Apart from that, there's not much more you can do, except for maybe disabling beat detector vis in stretched mode, and maybe disabling the spectrum analyzer/oscilloscope.
Config Tray -> Options:
Songticker -> Disable scrolling
Misc -> uncheck "enable beat vis"
Notifications -> never show
Drawers -> uncheck 'animate'
Other possibly relevant settings:
Prefs -> Plugins -> Output -> DirectSound -> config:
Buffering tab: enable cpu usage control
Prefs -> General Preferences
Priority class = idle
(If you have any problems with this, then move it back to 'normal' later)