Since White Mt. was the only one to answer your questions, I thought I would offer you a couple of quick answers to your queries based on my non-LDS, trans-denominational perspective:
First, In John 16:7 Jesus says, "But i tell you the truth: It is for your good that I am going away. Unless I go away, the Counselor will not come to you; but if I go, I will send him to you." If we work from the premise that The Holy Spirit is God (as Acts 5:1-4 and many, many others would affirm), and if one of the attributes of the Holy Spirit is omnipresence (as Psalm 139:7-10 would affirm), then why is it necessary for Jesus to "send" the Spirit when the Spirit is clearly already here?
Looking at the ministry of the Holy Spirit in the context of the entire NT, it becomes clear that their is an aspect of the Holy Spirit's ministry that is often referred to as "indwelling." The Holy Spirit, through some means that remains mysterious to this day, somehow ties himself to your soul & spirit, enabling you in wisdom, power and love far beyond your own abilities. It is very much an experiential reality, something you can actually feel.
Second, more practical question (and this one I do have some sort of opinion on, but I want to hear other views), can somebody be a Christian without having the Holy Spirit? And, if so, what are we to make of Paul's question to the Ephesians in Acts 19:2, "Did you receive the Holy Spirit when you believed?"
Yes, you can believe in Jesus without the indwelling of the Holy Spirit. As a matter of fact, intellectual belief in Jesus is prerequisite to being filled with the Holy Spirit in the NT. (Other rules apply for the indwelling of the Holy Spirit in the OT...) However, there is nothing in the NT that would lead us to believe that you must be filled with the Holy Spirit upon intellectual belief in Jesus. It is not automatic nor is it guarenteed. Belief in Jesus and being filled with the Holy Spirit are always presented as being two completely different and seperate acts throughout the NT. We equate these two different acts at the peril of greatly distorting the Word of God.
However, more to the point of your question - no, a person cannot be saved without being born again:
Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and [of] the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
Marvel not that I said unto thee, Ye must be born again. The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit. - John 3:5-8
But the hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship him. God [is] a Spirit: and they that worship him must worship [him] in spirit and in truth. - John 4:23-24
But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. And if Christ [be] in you, the body [is] dead because of sin; but the Spirit [is] life because of righteousness. - Romans 8:9-10