I've had this little project sat on my plate for nearly 6-8 weeks now, and having finally, almost reached the point of completion, I wanted to get this experience down where others might benefit and learn from it.

A little background, my Admin server (running SIMS, FMS, and SCO) was upgraded sometime last year or so to SQL2008, and though I didn't realise at the time, that this upgrade coincided with an increase in problems with our Symantec Backup Exec system.

We have Symantec Backup Exec 12 since 2008 on the server, with nightly backups to magnetic tapes. The main trouble we were getting was constant crashes from the Backup Exec Job Engine Service, and occasionally some of the others too. They would hang, refuse to start, stop restart and all sorts of errors.
An inspection earlier this year highlighted that our backup systems could do with upgrading. The tapes were barely large enough to hold the data, and they were reaching the end of their usable lifespan, and the tapes cost a lot to replace a whole set.
The decision was made just before the summer to move to Portable hard drives as a backup media to replace the tapes with, being cheaper, easier and much larger storage capacities.

The trouble starts
About half way through September, one of our backup tapes dies, so we attempt to setup backing up to our portable drives. It didn't work well. The services kept bugging, and we couldn't modify the Job setup.

Some calls to Symantec and to Capita, and I finally get the answer that "Backup Exec 12.0 does not support SQL 2008. You must upgrade to v12.5 or later to fix the issue."

The incompatibility causes some of the service instabilities, as well as you being unable to modify existing jobs that you've created. The creation part will work fine, but once created Backup Exec will crash whenever you try to open the properties.

The solution
If you have bought into a symantec support package (not a clue which one, but apparently there is one or several that allow upgrades like this included in the price of the support), then you should be able to upgrade to version 12.5 for free. Chances are, if you are using software 3-4 years old, then you probably don't have this package (since anyone with it probably upgraded as and when prompted to do so each time!).

I opted to upgrade fully to Version 2010 R3.

Getting the licenses was relatively painless; you order the license from a reseller (you have to be very specific as to which license you require; there are multiple types, flavours and versions depending on OS type, 32/64bit, then standard/server/personal/enterprise/managed etc etc.)
Then you get an email with your license details.

This serial needs to be registered on the symantec licensing portal, which you should already have an account for if you have V12 or similar aged software. If you don't know it, then give them a call.
I discovered my software was actually registered to the personal email of an engineer for the old external support company my school used! An email bounced a few times between myself and symantec (including purchase details and license details I had on those purchase details), and the licenses were transferred back to my account and the schools email address.

Once entered, you then have the option to download your new shiny software, and I'm going to assume from this point you can work out how to operate a download page....

A few notes on the installation process:
-As your messing with SQL, it's best to get a full backup done before you begin messing with software, installs or settings. Windows includes it's own Backup utility, under system tools that will work if you cannot get backup exec to play ball one last time. It's better than having no backup!
-The download is supplied as a multi-part ISO and a CMD file that combines the 4 parts into a single DVD-sized ISO file. make sure you select the .CMD file too! I nearly forgot to include it in my download.

-You have 2 options for the upgrade; remove the old one, and install the new one afresh, and setup all jobs, catalogs, media sets and devices from scratch.
Install as an upgrade. I found this to be painless, completely. It carried over all my old jobs, catalogs, media sets, the lot, and I had it set to backup within 10-15 minutes of completing installation. This was by far the easiest option IMO.

-Before beginning the upgrade, make sure all your users are off any managed databases (SIMS, SCO etc). Part of the process installs some SQL stuff that likely won't play well if there's people using databases at the same time.
-You need to stop all Backup exec services (set to manual and stop them) AND THE SERVICE "SQL SERVER(BACKUPEXEC)." Dont miss this one!

-Check your admin logins and details. I've had a problem with mine being unable to backup system state as access denied. There's a login for this that's not right somewhere, haven't figured out which it is yet.

-There's a good chance, the first backup you run will popup a box at the end asking you to eject the media and press the OK button. There's a tickbox option at the bottom of this window, that allows you to have this box automatically OK itself. You will want to set this, otherwise your Job will continue to run despite finishing all other tasks happily.

Symantec Backup Exec 12.0 does NOT support SQL2008. It will crash, things won't work, it will cause you problems.

Solution: Downgrade to SQL 2005 (NOT RECOMMENDED!), or pay for upgrade to newer version. At time of posting, all newer versions (12.5 or later) should solve the problem, but you'll most likely have to pay for it, so buy the latest version anyway, which should be 2010, though I believe 2012 is on the verge of being released, or released very recently.