[fel64]

The #1 thing to do is to backup regularly. You can’t be 100% sure that you’re safe – but if you have a backup it’s not a particularly big deal. If you use WordPress use the backup plugin for that and include the bb tables too.

If someone else users your computer and you’re still logged in, then they have access to everything you have access to. You can’t do much damage through bbPress; deleting posts etc. isn’t permanent, for example. What you should be concerned about (if you reckon someone nasty is going to get to your computer at all) is your FTP, database and hosting admin access.