The mass of a average sized virus is about 1.5 femtograms (1.5*10^{-15} g)and viruses are made up primarily of protiens. Some coat themselves in sugars or other substances but these tend to be the bigger ones. Protein is roughly 70% carbon, 12% nitrogen, 10% oxygen, and 7% hydrogen by mass. We then break down the mass of the virus into these parts and get:

C: 1.05E-14g
N: 1.95E-16g
O: 1.50E-16g
H: 1.05E-16g

I will work through the remainder of the calculation for carbon and then just give the results for the others. The process should be redily duplicatable.

Consulting a periodic table we find that carbon is 12g/mol = 12 g / 6*10^23 atoms.

1.05E-14g / ( 12 g / 6E23 atoms ) = 1.05E-14g * 6E23 atoms / 12 g
The units cancel and we multiply the terms in the numerator:
= 6.3E9 atoms / 12 = 5.25E8 atoms = 525,000,000 atoms of carbon.

Repeating for the other elements we get:
C: 525,000,000
N: 8,360,000
O: 5,630,000
H: 63,000,000

For a toal of aproximately 600,000,000 (six hundred million atoms)