To completely ignore Doctor X' message and put it in other words:
1.) You can't "downgrade", i.e. a reinstallation is required.
2.) Backup everything. Easiest: Clone your harddrive to another harddrive with Carbon Copy Cloner.
3.) Clean install Tiger from the original installation disks that came with the computer.
4.) Bring back important data from the clone.
Some information can probably _not_ be brought back, i.e. Mail.app upgrades the database when upgrading to Leopard. I'm not sure whether you'll manage to bring all those mails back. Similarly, it's possible that iCal and AddressBook don't handle the newer databases well.
A whole other question that arises: Are the problems you're having now solveable? Maybe we can find a solution that _doesn't_ involve going back to Tiger?