I think this should be a fairly easy question to answer, for those of you who know about this kind of stuff, and I've gone on google and tried to resolve this myself, but then I just got confused.

I have a video file that I took with my microscope camera at work, and the filesize is huge. To give you an idea, the video is a minute and a half long, and it comes out to about 2.5 gigs. I think that the obese file size is causing it to play back slowly, because my work computer just can't keep up with all the information, and it looks a bit jerky in some spots when it plays. So, what is the best way to bring this file down to a manageable size? Is there a certain program I should be using? I want to do this at work, so pirating a program is out. I have to either use a free program or convince my supervisor to buy one. Any suggestions?