Random 100% Finder CPU Usage On One Account
First:


consult the Utilities -> Console.app and check your logfiles, for some idea. ;)


Next:


Activity Monitor (also in Utilities) may help shed some light and can tell you what code Finder is busy with and what else is happening at the same time.


Look over the routines when you sample the Finder to see if they make any sense (or at least Finder gets caught in the same code each time to know you have one cause and not many)
What processes are also getting CPU when this happens? 
Does this happen when all networks are disconnected? 
Are disk activity and network levels higher or lower than normal when this happens? 



If Console does not help, you can trace all the network and file usage to see if you can snoop at what Finder and the system is doing:


open Terminal.app
in its preferences change the scroll-back buffer to 10.000+ lines
and enter the next command
sudo fs_usage   #you will need enter the admin password
after a while (10-20 seconds) you can interrupt with CTRL+c
now scroll back and try get some idea - i understand than it is hard, because you will got hundreds lines what are normal...


This will show what files are currently used in the whole OS - maybe you will get some idea, what's happening.

maybe:


HDD error in several sectors - (try check smart-status for your HDD)
bad QuickLook plugin?
remote HDD (iDisk?) timeouts?


UPDATE:

First - if your wife's account is OK - thats can mean,


the problem is "somewhere" bellow your $HOME dir, so it is probably not in /System or /Library, or
the problem is "somewhere" in /System or /Library, but caused by your login (StartupItem or LaunchDaemon or some your Preference setting what use Finder for some work - for example TimeMachine)


Some guessing (but honestly - these are not "advices" only pure guessing)


uninstall PathFinder.
check $HOME/Library/QuickLook folder. if anything is here, try move into ~/Desktop (for temporary) and relog - if not help, move them back - your should relog when moving plugins.
check /Library/QuickLook (let there here only QBQL.., iWork) - don't trash them, only temporary move to desktop and relog.
do you installed any Finder-plugin/hack - something?
try disable TimeMachine - if it is enabled
try eject (umount) all networked drives, and usb drives
have you BootCamp especially with NTFS? - Try exclude it from Spotlight, TimeMachine etc...
try exclude everything from Spotlight (this probably will not help, but for "sure" - your mdworder should stop indexing) (in the SysytemPrefs -> Spotlight -> Privacy - add your whole hdd and bootcamp too.)
disable all sharing (SysPrefs->Sharing)
download Onyx and remove all Caches. (this will cause increased CPU usage while rebuild cache again, but only for short time)
remove ALL .DS_Store (hidden) files - don't worry, they're not essential. Sure exists some GUI tool for this job. If don't find any, you can try this Terminal command.

find "$HOME" -name .DS_Store -print0 | xargs -0 echo



the above is for "dry run" - you can check what will be removed, if satisfied, change the echo to rm


if none of above help (and probably will not :=() have no idea...


bmike? any other idea?