Bicycle Tutor

Hi,
I've got an old mountain bike (10 - 15yrs) that I'm playing about with. I was trying to remove the stem so that I could get a threaded to threadless converter and fit a new stem. However, rather than doing what I should have done and read the tutorials (http://bicycletutor.com/overhaul-threaded-headset/.), I just went ahead and undid the the tightening bolt. Now, no amount of pulling or twisting seems to get the stem any closer to coming out. Is there anything I can do? Could it just be so rusted that it's beyond hope?
Any advice would be much appreciated.
Thanks

My guess is that the stem probably is rusted. This can happen quite easily. As far as the tutor that you linked to, it says:

To remove the stem, simply loosen the stem’s tightening bolt far enough so you can give it a light tap with a hammer. This will knock the inner wedge loose and allow the stem to slide up out of the fork.

For my experience, you'll have to try twisting the handlebars around enough so they move independent of the fork/front wheel to free it. Soak some WD-40 or Liquid Wrench in the stem thread to help. If the stem is free and the tightening bolt is loose enough, it should come right on out with the light tap of the hammer, if it even needs that (mine have always come right out - to the point that I have to watch it in even adjusting them).

I did try a bit of twisting and pulling but maybe I could give it another go. As for the light tap with the hammer - I would try it except that the tightening bolt doesn't seem to be doing anything any more other than sitting in the hole looking pretty. After I took it out it didn't seem to have anything in there to screw back into.

If the bolt is out, then the stem is rusted into the fork.
Flip the bike over and put some liquid wrench or other rust breaker in the hole at the bottom of the fork where the two legs come together. This is the same as dripping liquid wrench at the top where the stem goes in to the fork, but I think you can get more in there by dripping it in from the bottom.
Let it soak a few hours then put a wood block on top of the stem and give it a few firm hits with a hammer. You're trying to break the bond between the stem and the fork. Be careful not to drive the stem down so the wider parts start hitting the top of the headset. If you can get it to move at all, you should be able to pull it out with some twisting and pulling. Try repeating this process if it doesn't work the first time.
FYI - this is why you should put grease on your stem and seatpost when you insert them intot he frame.

If it helps, when I freed the stems on these bikes I have, I straddled the front wheel, held it in place with my legs (the best I can) and then did the twisting on the handlebars. It'll take a little effort (and a lot of rust breaker with time to soak), since you're trying to break the parts free for the rust.
Patience and effort will get it if it's not TOO rusted.... Good luck.

Cheers guys, shall try rustbreaker and brute force. Will probably end up using this bike as a bit of a project bike to take apart and fiddle around with - so nothing to lose at least.