Try These for a Fix!
Follow these steps in the given sequence:
-- Fix1 --
1. With nothing even remotely related to Yahoo! (like YMessenger, Trillian, other Yahoo services like Yahoo! Greetings etc.) sign in to Yahoo! Mail using the "ID" you are having problem with through your browser. Then Sign Out and then Sign Out Completely.
2. Now try your YMessenger with the same "ID" and see if that works.
If Not, Do this for all the browsers that you might have used to access Yahoo! services:
-- Fix2 --
1. Clear your Internet Cache. For IE go to Tools >Internet Options > Delete Files > Check "Delete all Offline content" > OK.
2. Clear all your Cookies. For IE go to Tools >Internet Options > Delete Cookies.
3. Now go to <DRIVE>:\Documents and Settings\<USERNAME>\Cookies
where DRIVE is where you have your Win XP installed and USERNAME is the username you are logged in with.
4. Delete all the files including the Cookies (*.txt) there. NOTE: You will not be able to delete index.dat. Leave it as it is!
5. Now run Disk Cleanup from Accessories > System Tools and cleanup everything it suggests you to. NOTE: These files are completely save to remove off your system. In any case, at least delete all Temporary Files.
6. Now manually try to delete all the files present in DRIVE:\Documents and Settings\USERNAME\Local Settings\Temp. If you can't delete some files leave them and delete all others you can.
Now try your YMessenger with the same "ID".
In all probability, everything should work out fine.
If you still face the same problem, don't loose hope and try this:
-- Fix 3 --
1. Try to log into YMessenger using a "Different ID".
If the things repeat themselves and you get logged off immediately then it all points to the fact that you have some problem with your internet connection. Unfortunately, I can't help in here unless I get to know your OS environment better. Anyways, Please check your connection settings, shut down any firewall and if you use a proxy check that it's working fine.
However, all this is very unlikely to happen as you mentioned that you do get connected using Triliian which I suppose is using the same internet settings. Please, check that.
2. If the "Different ID" does work, Sign Out and close YMessenger.
3. Now go to DRIVE:\Program Files\Yahoo!\Messenger. Now if you use an older version of YMessenger that allows you to save any offline messages then Rename "StoredIM" to something like "~StoredIM".
4. Now go to DRIVE:\Program Files\Yahoo!\Messenger\profiles\ID\Archive and Rename "Messages" to "~Messages" where ID is the ID you are having trouble signing in with.
5. Now, check if you can login using YMessenger via ID successfully.
If you do, then someone had sent you a "boot code" that caused you all this trouble.
6. To fix this, first determine which folder "StoredIM" or "Messages" is causing you the pain and copy that folder in the same directory using right-click press -> drag -> drop -> select "copy here".
7. Rename the two folders to the names they originally had and delete all the files present in the folder causing the trouble.
8. Now if "StoredIM" was causing the problem move one Offline Message (*.hst) at a time back from "Copy of ~StoredIM", or If "Messages" was causing the problem move one Message Folders at a time back from "Copy of ~Messages".
9. Now try to Connect your YMessenger to check if it's causing the trouble.
10. If Yes, delete it and repeat the process.
I hope this should surely help you.
Do let me know if it does or does not...