That may be because FPSC has something held in memeory.
Ive had that kind of problem when trying to reinstall.
FPSC can be tricky to work with because of this in many ways.
Not sure if it will help at all - it may or may not but you could try going into the temp folder - My documents - local settings or something like that on most machines under Win XP. Find the FPSC temp files and delete them. Try not to delete any files not associated with FPSC if you can help it. FPSC files normally have names starting with DBP.
Reboot and dont do anything with the FPSC program - have a look in the temp folder again just to check the situation - It usually will allow you to delete all the FPSC temp files but the last one created which is normally in use by the system. So you should still have perhaps one left there. I doubt you will be able to delete that last one.
If all looks OK then try the reinstall from the original download you got and see what happens.
If that dont help not sure what to suggest.
Good luck with that.
"I am and forever will be your friend"