I've been trying to upgrade from my 10.5.8 to 10.6.3. I have the disk for snow leopard but I can't find the disk that came with my computer. when I insert the disk and try to upgrade, an error message displays saying "Mac OS X cannot be installed on this computer" and quits the installation. I understand restoring my hard-drive could help (I've already made a back-up) but is there a way to do this without the disk that came with my computer? or any other solutions to this problem?
Make sure that there's not a directory called backup or backupd on your 10.5.8. If there is then SL won't install until it's gone. Make sure you have a backup somewhere else and delete the directory and try again.
If the backup directory is not the issue, check how much free space you have on the 10.5.8 disk.
Correct. You should be OK. You should do a little research for apps that aren't compatible or need later versions. Here's one guide to get you started.