Backing up files

Can somebody just give me a pointer here please ?
I want to backup some of my video clips in case of future use. The avi files are much larger than the mpg files I create from TMPGenc, but I cant seem to import the mpg files back into Windows Movie Maker.
Should I be able to import mpeg files created by TMPGenc into Windows Movie Maker ? I try clicking on 'Import video files' , but it starts, then just seems to hang. The green bar races across the 'importing files', but then seems to hang. The files from TMPGenc seem to be .mpg files (though TMPGenc says they will be mpeg2) ,both of which WMM says you can import. The file Im testing is only 200k so isnt that large.

Thanks.