Hi Losh, Here is a video series that uses Xmodem to upgrade the IOS of a 2610XM router. It should work very similar with your 1841.
In part 1, he goes into too much talk on how to set the baud rate, Just grasp and do the main procedure and you can move to part 2 which is where the core tutorial is. Just make sure you return the baud rate to the default on both your PC and the router after your done.
Note: I apologies for some censored words that the guy uses in his talk. The video is useful though, I couldn't find a better one to post. Other videos where for Cisco switches. If others here have better links, please post.