This is in response to sahabia's request for clarification on the first two lines:

The chariest maid is prodigal enough
If she unmask her beauty to the moon:

The term "chariest" is the word that indicates she is virtuous (a virgin). It means "modest". The word "prodigal" means "giving or yielding profusely". It is because of the close pairing of these two words, along with the general gist of Laertes speech, that the lines can be read as speaking of her sexuality. She may be a chaste, modest maid (read--virgin) but she is immodest (prodigal--read too free or open with her sexuality) if she "unmasks her beauty to the moon."

As for the second part, the unmasking to the moon, there are several layers of possible meanings. The moon, of course, is a significant reference because it is suggestive of female sexuality. The first, literal meaning of unmasking to the moon, is to show herself in the dark, and by this, I am not talking about her face. A maid who is exposing her beauty (body), even in darkness, is not (according to Laertes) as modest as she appears. It is not whether she is exposing her body to the moon that will cause her lack of virtue, it is what she does under the cover of darkness that is the problem. This leads to the second layer. Laertes suggests that no woman is modest if she does deeds in secret, even if she appears to be in the light of day.

Thakkar's comment on the times is excellent. I'd add this reading:

The chariest maid is prodigal enough
If she unmask her beauty to the moon:
Virtue itself scopes not calumnious strokes:
The canker galls the infants of the spring
Too oft before their buttons be disclos'd:
And in the morn and liquid dew of youth
Contagious blastments are most imminent.

The first two lines speak of modesty and extravagance, with sexual undertones--a modest maid is not modest if she unmasks her beauty to the moon (reveals herself in the dark). My reading of the third line is that virtue doesn't include or target slander which I think means that people don't slander the virtuous because they don't do anything to attract it. Finally, the last four lines are about tender young buds getting the blight before their petals have unfurled, an allusion to the potential perils facing a young virgin who has not yet blossomed into womanhood. The idea here that "contagious blastments" are lurking in the "morn and liquid dew of youth" suggests that Ophelia's very youth may invite the blight (metaphorically) because it is that and her beauty that may bring about her ruin--the bud is a virgin, a blighted bud is soiled. Ophelia is in danger of being spoiled because she has attracted Hamlet and because, being young, and damp, she is susceptible to corruption, as are all young women. According to Laertes.