Dai-Guard is a 26 episode long Real Robot anime about the 21st Century Defense Security Corporation, who develop things for self-defense and security, who use the robot Dai-Guard created by the government to battle the Heterodyne who appeared 12 years prior, who only showed up a single time, with no need any more, was kept as an attraction for the 21st Century Security Corporation as well as mascot. In the present day, at an expo showing off newly developed tools made by the Corporation, a Heterodyne shows up and causes havoc, which the army can do nothing against. Akagi, a regular desk drone there launches the Dai-Guard in order to protect the people and stop the Heterodyne.

I've been watching the show and I really like it. The theme's awesome too, just listen! And the TV Tropes description is much more enticing than my lame one.