I am running Windows XP pro XP2 and Trados 6.0 and 6.5 Freelance.
You mustn't run two Trados versions simultaneously - I assume what you meant is that both versions are installed on your computer. If so - why?
I reinstalled Trados 6.5.5 and it is still happening.
One of the problems associated with having two Trados versions installed is that you probably confused them, for example, by attempting to use Workbench 6.0 while T6.5 is registered in your system.
Which version are you using now (full version please - go to Help - About... to check)?
Did you register the application before using it (Start - Programs - Trados 6.5 FL - Register Trados 6.5 FL)?
I would strongly suggest to get rid of Trados 6.0.