Is there a Christian basis for using garlic, crosses, and holy water against evil?

In Catholic Tradition and culture, vampires only exist in scary novels/movies, and garlic is only used for seasoning delicious meals.

However, crosses, holy water, and various other holy objects and gestures can be ordained by the Church as sacramentals.

Sacramentals are material objects, things or actions (sacramentalia) set apart or blessed by the Roman and Eastern Catholic Churches, the Eastern and Oriental Orthodox Churches, the Church of the East, the Anglican Churches, the Independent and Old Catholic Churches, and the Lutheran Churches to manifest the respect due to the sacraments, and so to excite pious thoughts and to increase devotion, and through these movements of the heart to remit venial sin, according to the Council of Trent (Session XXII, 15).

Sacramentals are sacred signs which bear a resemblance to the sacraments. They signify effects, particularly of a spiritual nature, which are obtained through the intercession of the Church. By them men are disposed to receive the chief effect of the sacraments, and various occasions in life are rendered holy.

The characteristics of sacramentals

Sacramentals are instituted for the sanctification of certain ministries of the Church, certain states of life, a great variety of circumstances in Christian life, and the use of many things helpful to man. In accordance with bishops' pastoral decisions, they can also respond to the needs, culture, and special history of the Christian people of a particular region or time. They always include a prayer, often accompanied by a specific sign, such as the laying on of hands, the sign of the cross, or the sprinkling of holy water (which recalls Baptism).

Sacramentals derive from the baptismal priesthood: every baptized person is called to be a "blessing," and to bless. Hence lay people may preside at certain blessings; the more a blessing concerns ecclesial and sacramental life, the more is its administration reserved to the ordained ministry (bishops, priests, or deacons).

Sacramentals do not confer the grace of the Holy Spirit in the way that the sacraments do, but by the Church's prayer, they prepare us to receive grace and dispose us to cooperate with it. "For well-disposed members of the faithful, the liturgy of the sacraments and sacramentals sanctifies almost every event of their lives with the divine grace which flows from the Paschal mystery of the Passion, Death, and Resurrection of Christ. From this source all sacraments and sacramentals draw their power. There is scarcely any proper use of material things which cannot be thus directed toward the sanctification of men and the praise of God." (CCC 1668-1670)

The Rituale Romanum clearly defines the order in which a priest or deacon may bless a sacramental. Sacramentals such as crosses, holy water, and blessings are employed during most rituals in the Catholic Church. These rituals range anything from Baptism (which itself is a protective exorcism) to weddings to demonic exorcisms.

About exorcisms...

Yes, the Catholic Church performs hundreds of real exorcisms every year. Demonic possession in the Church is very real, and sacramentals have been used since apostolic times to battle evil spirits.

But don't be fooled by over-blown Hollywood concoctions of "head-spinning" myths and "green vomit" superstitions about how the Catholic Church supposedly performs exorcisms.

The most realistic and accurate portrayal one will find of how the Catholic Church performs the rite of exorcism is The Rite with Anthony Hopkins.

When the Church asks publicly and authoritatively in the name of Jesus Christ that a person or object be protected against the power of the Evil One and withdrawn from his dominion, it is called exorcism. Jesus performed exorcisms and from him the Church has received the power and office of exorcizing. In a simple form, exorcism is performed at the celebration of Baptism. The solemn exorcism, called "a major exorcism," can be performed only by a priest and with the permission of the bishop. The priest must proceed with prudence, strictly observing the rules established by the Church. Exorcism is directed at the expulsion of demons or to the liberation from demonic possession through the spiritual authority which Jesus entrusted to his Church. Illness, especially psychological illness, is a very different matter; treating this is the concern of medical science. Therefore, before an exorcism is performed, it is important to ascertain that one is dealing with the presence of the Evil One, and not an illness. (CCC 1673)

The Catholic Encyclopedia says about sacramentals:

One of the most remarkable effects of sacramentals is the virtue to drive away evil spirits whose mysterious and baleful operations affect sometimes the physical activity of man. To combat this occult power the Church has recourse to exorcism and sacramentals. Another effect is the delivery of the soul from sin and the penalties therefor. Thus in the blessing of a cross the Church asks that this sacred sign may receive the heavenly blessing in order that all those who kneel before it and implore the Divine Majesty may be granted great compunction and a general pardon of faults committed.

Its important to recognize that sacramentals, such as crosses and holy water, are not magical. They are not idols. They are simply material objects that are connected to the heart of the Church (the 7 sacraments instituted by Christ) via the ordaining power of the Holy Spirit and are entrusted to the priesthood of all believers for both edification and protection.

All sacramentals draw their power from Jesus Christ...who is God.

And from my personal experience, garlic is only effectively used when its sprinkled on buttered toast.