i don't know if you use excel or not, but it wouldn't be that hard to set up a personalized sheet with forumlas to automatically change your inventory when you enter in what you use.
I'm willing to share my set up, if you want to see it. I'm not sure how to do it in a thread though. [B)] If you email me, I can send it in an atatchment.
I use the program that Grant mentions above (www.penventory.com) and like it very much. If you have any questions after you look at it ask and I'll try and answer them.