Pre-requisities: Working system on 10.5.4!
1. Download 10.5.5 from here
2. Mount DMG
3. Run Terminal
4. Type 'sudo passwd' and enter your password
5. Set your root password (and confirm)
6. Type 'su root' and enter your password
7. Type 'while sleep 1 ; do rm -rf /System/Library/Extensions/AppleIntelCPUPowerManagement.kext ; done' (the terminal window should just 'freeze' (it's processing a loop))
8. Run the updater (MacOSXUpd10.5.5.pkg) from the volume you mounted earlier. When it completes do NOT restart
9. Switch back to the Terminal window
10. Press Ctrl-C
11. Type 'exit' (and then again) to close terminal window
12. Switch back to updater and press restart
13. Party on down!
NOTE: When the system boots, you'll need to reinstall the video drivers, just drop the kexts back onto the kext helper and reboot when prompted.
PS Kudos to netkas for the handy command line!