I highly recommend the GNS3 (graphical network simulation) tool. http://www.gns3.net
It's a collection of several applications that each do their own part to make things work.
- Dynamips, the core program that allows Cisco IOS emulation.
- Dynagen, a text-based front-end for Dynamips.
- Qemu, a generic and open source machine emulator and virtualizer.
The caveat here is that you need a copy of the IOS image you intend to run. Suffice to say, you generally need a smartnet contract or CCNx login on Cisco site to obtain it. With a little searching though, you'll likely find it.
It's been a while since I fired up GNS3 so I can't say with 100% certainly that the 1800 model was supported. However many routers are, and the functionality for command sets would likely be good enough if the 1800 isn't in there.