Hi Bec
A 2.6GB file plus a further 960 odd MB file is massive for a machine to 'render' in windows movie maker, and I think you will find you have insufficient RAM for such a process. You can verify this by either downloading the gadget that monitors your ram and cpu here
http://gallery.live.com/liveItemDeta...5-157bd66ca5af
or by CTRL / ALT / DEL and view the same information in the task manager. So load up your files in Movie maker and check out the RAM usage. I would expect you are suffereing from insufficient RAM thus the poor playback. Rmember, using Vista will hover at close to 1GB doing nothing, which is not a problem normally, but will be if you start wanting to do video editing. I use Adobe Premiere Pro and even with 3GB it ocassionally complains about insufficient memory.
The solution maybe to convert these files to another file, however my knowledge in this area is limited.