You can not just MAKE an low quality file HD as the extra data is just not in the images(frames). Sure some programs upscale better than others, but I think your issue is mostly one that can not be avoided.
Even with a quality scaling the tomshardware logo looks like this at just 2 x the size
Free programs you can try are Handbrake ( http://handbrake.fr/ ) and MPEG Streamclip ( http://www.squared5.com/ ) but as nukemaster stated the SD files lack the information to be successfully upscaled to full HD. You could also look at Red Giant's Magic Bullet Instant HD 1.2 ( http://www.redgiant.com/products/all/magic-bullet-instant-hd/ ) but it is far from free ($99). http://www.videomaker.com/community/forums/topic/convert-sd-to-hd-is-it-really-possible