I have been working on upgrading a customer from Symantec Backup Exec 10d to 12.5 today. I hadn't done one that was this old for a while so I forgot to you have to be on at least 11d in order to do this. So I had to completely uninstall 10d in order to do a fresh install.
The uninstall went fairly well, but the install of 12.5 failed with some error codes I had to look up. Since I was working remotely I forgot you had to be in console mode in remote desktop in order to get it to work right, so I ran an mstsc /v server /console.
I re ran the setup only to be stopped again. This time it was SQL. There was a half way installed version of SQL 2005 Express from a previous vendor. It wasn't showing up in add and remove programs but it it was definitely there. Now I had to go through the registry and uninstall each registry key by running an msiexec command for each SQL key that was found. KB article 909967 explains all of this.
The problem is that one of these keys still won't uninstall, so it's time to call Microsoft.