Hrm, the error message you're getting usually indicates a problem between windows and eqgame.exe file. Do you still have the shortcut on your desktop that points to eqgame.exe (with patchme on the end)? If you do, do you get the same error message when you try to launch everquest that way? If you do get the same error, your eqgame.exe may be corrupt and you may need to re-install Titanium, or find eqgame.exe on the Titanium disks and copy it over.
The only other thing I can think of, is WinEQ2 requires DirectX 9. Vista ships with 7, 8 and 9 already installed, but XP automatic updates will install DX10 and remove 9.
You can download DX9 (Aug 2008 release) here:
This will not remove DX10 and you will still be able to play any games that require DX10 as normal.