For anyone that happens to run into the bug that causes the Adobe CS5 updater to complain about another non-existant instance (both using the automatic updater AND while attempting to manually install the updates) Post # 21 in the following topic solved it for me and others. You just replace 3 files.
http://forums.adobe.com/thread/650599?tstart=0#21
8. Go to folder "c:\program files\common files\adobe\oobe\PDApp"
9. Make a copy of UWA folder.
10. Copy the three files present inside folder "c:\User\[uSERNAME]\Temp\For_User\\UWA-Devbuild-signed " into "c:\program files\common files\adobe\oobe\PDApp\UWA" folder.
A Zip is attached to the post and this one.