problem when downloading HD video

I have an older Dell Dimension 2400 and am having trouble viewing HD video downloaded using FIOS. (football games)I usually download them straight to my external HD. When playing the content after download, the video is choppy and freezes every couple of seconds. the audio usually keeps playing without a hiccup. these files are relatively large at around 2gigs and take all day,literally, to download. I also have 2 other pc's and a ps3 on my wireless home network via fios router but they are rarely used. The computer is pretty much the same as when I bought it (no upgrades internally) any ideas?? Could insufficient RAM be the issue? I also use VLC media player to (attempt) to view..