I would highly recommend NOT installing via upgrade. Backup your stuff either with an external HDD or DVD's and do a full erase of Tiger and install Leopard on an empty formatted HDD. Too many times people are reporting problems after upgrading to Leopard and it's mainly due to using the upgrade path or Archive and install. Yes, the upgrade option is available but it works best from a new install of the older OS and then upgrade to the new OS. I understand you have important data to save but it will save you from headaches down the line if you just do a fresh install of Leopard on an empty formatted hard drive.