Are you using the slanted apostrophe like that? - you just need the straight dash - like this ' :
UPDATE w3t_Posts
SET B_Replies = '0'
WHERE B_Board = 'archive'
The favorites and reminders *might* not cause an issue for you. I'm just making you aware of potential problem areas.
Unfortunatley there isn't a real easy way to check that stuff. If it were that easy, then it would be a feature from the admin menu.
Usually what's needed is some kind of script which checks the stuff and removes where necessary. So if the post number in the favorites table doesn't exist, then it can be deleted too.
I just like to throw appropriate caution as someone new comes along, finds a thread like this and runs the query to delete stuff... which often creates problems for them.
A good guide, if you want to delete posts, take a look at the scripts in the admin section that perform this action and see what tables they are updating etc... That's a good guideline of what needs to be done, depsning on what's being removed/changed.