Looks like you're a long-time player of TF2... and that the Halloween update broke it for you.
One reason for such a message is that TF2 is already (or still) running. Perhaps you tripple-clicked to start. Check for hl2.exe in the Windows TaskManager and end it. Then try TF2 again.

If that wasn't the problem, validate the steam cache files from the property dialog (The "Local Files" tab).
If you've done that and it still doesn't work, try uninstalling TF2, then going in through Windows Explorer and removing the Team Fortress 2 folder (might be leftovers). Then your next download(would take a while) will be 100% clean and should work without any issues.

If you've still done all that and it doesn't launch, the Halloween event might be nearly over by the time Valve gets to your bug report(you submitted one, right?). I hope you don't miss out.

My Bro and I both have accounts, His is working, mine isnt

I didnt triple click, so I might Uninstall it, then re-install it
Thanks for the help though ^^