I think Critters reply about this, referring to someone else's reply, may have solved it for me. I noticed I had a thumb drive plugged in and when I ejected the thumb drive I was able to once again open TurboTax without it saying enter activation code and then saying "All available activations for that code have been used. Error code 603."
I don't know why having a thumb drive inserted in a USB port--which has nothing whatsover to do with TurboTax, no content on the drive related in any way--clobbers the log on security, but apparently it does. When the thumb drive is in the usb port, the mysterious problem occurs and TurboTax is inaccessible. When the thumbdrive is ejected TurboTax can be opened normally.
Apparently having the thumbdrive plugged in confuses the software security somehow. It also confuses the user since that is not an obvious cause/effect relationship to the typical end user. (Or to TurboTax support-- maybe they should put that possible solution in their knowledgebase.)
