Seen that before!
A bit of advice DON'T install IE 5.5 it hoses up lots of things.
Sounds like index.dat in the \Windows\Cookies folders is corrupted.
To fix you must delete all the cookies AND the index.dat file.
You will probably have to strip the Attributes from the index.dat file to be able to delete it.
First just go to that folder and open it and hit Ctrl and A to select all files and then Shift and DELETE to delete them and bypass the recycle bin.
If it all deletes then close all windows and restart.
If not then:
Restart the computer in MS-DOS mode and navigate to the folder probably will be C:\Windows\Cookies
Type "CD.." to get to the root of C: drive.
When you get to a prompt like this
C:\
Type "CD C:\Windows\Cookies"
And then "Attrib index.dat"
You will get a response like "Index.dat s "
Strip the file attributes from index.dat by using the Attrib command again but adding a minus sign to the file attributes.
Probably you will need to type "Attrib index.dat -s"
File attributes can be h, s, or t so possibly you might need to type "Attrib index.dat -h -s -t"
After index.dat is stripped of attributes type "delete index.dat" and "dir" to confirm it is gone.
After that you type "CD.."
And at the C:\Windows\ prompt type "win"
After that, connect and log back in to The Firing Line and start rebuilding your cookies.
That will erase all you Internet Explorer cookies so be sure you have all your passwords BEFORE you do it.