Question about grabbing/modifying packets

["Quasar" <quasar () speakeasy net>]

Goal: 
To be able to watch for certain packets, edit them, place them back in the stream so the application is unaware that 
anything has been changed, ie the sender or anything like that.

Possible implementations that I can think of:
place linux machine inbetween me and the internet and write some kind of program to watch for those packets, change 
them if need be, and have it forward the packets to this machine

write a low level NDIS or TDI driver in windows
(dont have any experience doing either of those other than ONLY forwarding packets with a linux machine between me and 
the internet using iptables)

Is there any links anyone can provide on how this could be accomplished, or am I heading in the right direction?  Also 
I am wondering how I can stop the stream, maybe stick it in a buffer or something while i work on that packet then 
re-insert it and re-enable the stream?  Anyway I'm new to this and figured the veterans might be able to help.
Thanks in advance-