Let the rate at which Owen cleans windows be \( r \) windows per minute. Therefore, Greg, who is 1.7 times faster than Owen, cleans at a rate of \( 1.7r \) windows per minute.
When they work together, their combined rate is the sum of their individual rates:
\[
r + 1.7r = 2.7r \text{ windows per minute.}
\]
They clean 100 windows together in 39 minutes, so we can set up the equation:
\[
2.7r \times 39 = 100.
\]
Now, we can solve for \( r \):
\[
2.7r \times 39 = 100 \\
2.7r = \frac{100}{39} \\
r = \frac{100}{39 \times 2.7}.
\]
Calculating the denominator:
\[
39 \times 2.7 = 105.3.
\]
Now substituting this back in, we have:
\[
r = \frac{100}{105.3} \approx 0.948.
\]
This means Owen can clean approximately 0.948 windows per minute. Next, we need to find out how long it would take Owen to clean 100 windows by himself:
Let \( t \) be the time it takes Owen to clean 100 windows:
\[
r \times t = 100 \\
0.948 \times t = 100 \\
t = \frac{100}{0.948} \approx 105.5 \text{ minutes.}
\]
Therefore, it would take Owen approximately \( 105.5 \) minutes to clean 100 windows by himself.
Rounding to the nearest tenth, the final answer is:
\[
\boxed{105.5} \text{ minutes.}
\]