Not sure what you mean when you say Upgrade to Leopard.
Upgrades is not always the best solution. Often, Archive/install or (backup)-Erase-Install is better to get a clean system. You may simply have components on your system that don't work so great with Leopard.
Try to boot your Mac by holding down Shift after you hear the chime and releasing it when you see the spinning wheel.
Does that help? This is "Safe mode", so you shouldn't use your Mac and may get errors when it starts, but just reboot to make things normal.
Startup items that boot with the system are in /Library/StartupItems. You can make an archive of an item or all of them here, then remove them, keeping the archive safe, and reboot. If you find you don't need it, hey. If you find you needed it, unpack the archive back in the folder and reboot.
Try to boot your Mac without your startup items by holding down shift IMMIDEATELY as you click your user account icon (it may be easier to turn off automatic login if you have that enabled. Remember to know your password) to make this work. If you enter both, just hold down shift immideately after letting go of the return key or as you click the login arrow with the mouse.
Does this help any? To remove items and avoid them starting automatically with your Mac you can right click on their icon on the Dock or if not there (and to see the master list), go to your Account in system prefs and use the startup objects tab.
Try to repair Permissions using Disk Utility.
even my shut down is slow