Unchecking all checkboxes (or after a certain checkbox) function

[Death Slaught]

I play a game that when your backpack fills with items (the limit is 45) you must choose items to discard by unchecking their box. This is very annoying and time consuming so I was wondering if it were possible to create a Javascript function to be saved as a bookmark, and when used it prompts for the checkbox to start at (so it won't discard items that I want to keep) and discards all items after that point. So my main question is, is it possible for Javascript to manipulate a webpage in this way?

Here is part of the code used:

<FORM ACTION = maze.php METHOD = post name=battle><input type=hidden name=discard value=1>  
             <br>You cannot carry all of these items!
             Select 45 items to keep.
             The rest will be discarded.<table><tr><td><input type=checkbox name=Item0 value=1 checked></td>
               <td>Forest Sloth Fang (72)</td>
               <td>animal part</td><td><input type=checkbox name=Item1 value=1 checked></td>
               <td>Ridiculous Refreshing Potion (20)</td>
               <td>Restores 5000 HP and 5000 Mana</td></tr><tr><td><input type=checkbox name=Item2 value=1 checked></td>
               <td>Ridiculous Refreshing Potion (20)</td>
               <td>Restores 5000 HP and 5000 Mana</td><td><input type=checkbox name=Item3 value=1 checked></td>
               <td>Ridiculous Refreshing Potion (20)</td>
               <td>Restores 5000 HP and 5000 Mana</td></tr><tr><td><input type=checkbox name=Item4 value=1 checked></td>
               <td>Ridiculous Refreshing Potion (20)</td>
               <td>Restores 5000 HP and 5000 Mana</td><td><input type=checkbox name=Item5 value=1 checked></td>
               <td>Ridiculous Refreshing Potion (13)</td>
               <td>Restores 5000 HP and 5000 Mana</td></tr><tr><td><input type=checkbox name=Item6 value=1 checked></td>
               <td>Scroll of Antaria Warp (2)</td>
               <td></td><td><input type=checkbox name=Item7 value=1 checked></td>
               <td>Keen Claymore</td>
               <td>huge, 20d12 damage, +6 attack</td></tr><tr><td><input type=checkbox name=Item8 value=1 checked></td>
               <td>Keen Claymore</td>
               <td>huge, 20d12 damage, +6 attack</td><td><input type=checkbox name=Item9 value=1 checked></td>
               <td>Keen Claymore</td>
               <td>huge, 20d12 damage, +6 attack</td></tr><tr><td><input type=checkbox name=Item10 value=1 checked></td></tr></table>

Thanks,
{\_/}
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[acoder]

@Death Slaught
Yes, GreaseMonkey in Firefox and User JavaScript in Opera. There are also alternatives for other browsers.