Since they are different resolutions they will need to be re-encoded. If they are wmv, an easy program for encoding and joining them is Windows Media Joiner. It has a 15 day trial.
http://wmj.fronoh.com/Content/WMJ/
You can also try converting them with the DivX Converter and then joining the files with VirtualDub. Just make sure to use the same encoding settings on each separate part.
It would help to know what format your files are in.