Hello Fred and welcome to the community!
Can you please describe what you've done thus far?
Have you tried to rename the following file by adding a .bak extension at the end of the filename.
Windows XP
C:\Documents and Settings\All Users\Application Data\FLEXnet\adskflex_*_tsf.data
Vista/Windows 7
C:\ProgramData\FLEXnet\adskflex_*_tsf.data
When you restart Inventor it should automatically re-register / prompt for activation and as a result a new adskflex_*_tsf.data file will be created, potentially resolving this activation loop.
If you receive an activation limit reached error, please either send me a Private Message (click on my philip.s profile name and then "Send this user a private message") with the serial number + request code, or contact our License Assistance team for additional assistance.