Disclaimer: I did all tests on my late 2008 Macbook Pro Unibody 2.8 GHz model with NVidia chipset and graphic. Most of the following suggestions should apply to standard Macbook models and likely older generation as well.
First of all Intel Speedstep leads to dropouts and higher DPC latencies on small load! Unfortunately all tools that are supposed to manually switch Speedstep off don't seem to run on the late Macbooks (Pro) while on OS X you can use "Coolbook".
Your only way to make sure your processor is clocked high enough and not dynamically switching is to put up a constant load (like running your DAW pretty hot or running Prime95 at "Idle/Lowest" process Priority in the background). I will keep investigating if I can find a tool to switch Speedstep off.
Most importantly (to get rid of really bad DPC latency spikes):
Kill the process "KBDMGR.EXE"!
That's Apple's driver for controlling brightness and keyboard lighting via the function keys and setting tap options for the trackpad. It seems to have broken multithreading! You can also the CPU affinity of KBDMGR.EXE to CPU1 (not CPU0!) which will help decreasing DPC Latencies alot, but there will still be Audio dropouts.
Here's a small toolkit I put together that allows you to conviniently enable/disable Apple's "Boot Camp" tray application (KBDMGR.EXE) via an icon link and/or keyboard shortcut. Optionally it will switch the function of the F-Keys automatically for you depending on whether Boot Camp is loaded or not.
Furthermore it automatically turns Boot Camp's CPU priority to "Idle" and CPU affinity to CPU1 in order to turn down the bug induced DPC Latencies and prevent dropouts with Windows sounds and Media Player playback. Professional Audio users will find that only turning off Boot Camp will allow low audio latency usage. Installation instructions are included in the README.TXT for your convinience.
Boot CampED - Download Page
Turn off the Broadcom 802.11N WLAN driver via Device-Manager or update to the latest drivers via Microsoft Update Catalog.
Like on OS X the Airport module can lead to audio dropouts. The DPC Latencies produced by the Broadcom driver are less regular than the KBDMGR thing, alot higher in value. Best thing is to try for your own needs.
Update:Meanwhile a new Broadcom drivers was published via Microsoft's Update Catalog named "Broadcom - Network - Broadcom 4322AG 802.11a/b/g/draft-n Wi-Fi Adapter " (4322 is the chip used). This one comes with both low DPC latencies and finally the ability to use the full rate upto 300 mbit/s. Go get it! For safety you might still want to turn WLAN off during critical audio work though.
Change the graphic-card driver to "Standard VGA Driver" via Device-Manager or use RIVATUNER to enforce a fixed clock-rate and performance mode.
Update:The dynamic clock-rate switching happening with NVidia drivers in order to save power and keep temperatures low leads to extreme DPC spikes for each switch and constantly high DPC latencies when it settles in low performance 2D mode. RIVATUNER's "Enforce Performance Mode" option can be used to set the card to a fixed clock-rate. I recommend using "Low Power 3D" for audio work.
User of XP might think that they don't need this, but be aware that on XP the NVidia driver keeps running at highest clock-rates in "Performance 3D Mode" all the time. Via RIVATUNER you can switch to "Low Power 3D".
Turn off the ACPI compliant Battery driver via Device-Manager
This driver polls the battery for its current load status and produces a small, single, short spike exactly every 15 seconds. In my own tests I found that it doesn't seem to affect low latency audio performance. Furthermore turning it off will remove monitoring of your current battery status. But if you are running on power-chord anyway and want to make absolutely sure you can turn it off.
All other devices don't add much if anything to DPC latencies, but can savely be turned off if you don't need them (like Nvidia LAN, Bluetooth, Onboard High Definition Audio).
Attention: Removing the Battery while the power chord is connected results in permanently reduced CPU clock (downto the lowest clock setting possible). According to Apple this is done to prevent overloading the power-supply during heavy load as it needs the assistance of the battery from time to time.
