Here's a link analyzing this type of dream (other than the cow bit):
http://www.unclesirbobby.org.uk/dreamdictionary/intruder.phpI've also had persistent dreams of "an intruder" or "intruders", off and on since I was a child.
Personally, I think this is because I grew up with a father that was not very reliable, and often left me feeling "on shaky ground" due to his own emotional instability, and since then, I've just begun to realize the past few years that I've let people into my own "sacred space" who were not trustworthy, much like my biological father.
This is, at least, how I interpret this kind of dream, in light of the link's description...
Possible solution to this problem presented by the unconscious mind through dream: Take a second look at the people you have allowed into your own sacred space and inner emotional sanctum, and don't feel afraid to put up new barriers or maintain a distance for some time, if you (or I) think it would help give a healthier sense of security and stability.
The mind is always trying to communicate something through dreams, I think... and lately, my dreams have been pretty explicit about certain people I should re-evaluate my relationship with, for my own person peace of mind and ability to be more productive and creative.