You're correct that capitalizing Windows gives away the joke. Since the joke is the whole purpose of the sentence, leave it in lowercase. I'm sure everyone will forgive you for your laxness in the case of one proper noun.
You should use the verb is because, in the end, the word windows should be understood to refer to the operating system. So it should be kept grammatically correct for that reading. Also, you might consider changing "is the" to "is an." The singular article would reinforce the singular verb.
This isn’t really a joke as such, but it is a pun, centered around a not-often-seen meaning of the word Irish. The OED article on Irish has this in sense A.5.c (adj.):
colloq. (somewhat offensive). Of a statement or action: paradoxical; illogical or apparently so.
The speaker is presumably talking about boxing and thus a fist blow, but he measures its force in foot-energy. Therefore he adds “if I may say it like that without coming off too illogical, giving foot values to hand blows”.
As Compro01 points out in the comments, the foot-energy reference is most likely to the Imperial unit foot-pound, which measures energy (“the energy transferred on applying a force of one pound-force (lbf) through a displacement of one foot”), corresponding to the metric joule.
This actually makes the pun a double-pun, as it were: the unit of measurement relates to the foot as a unit of length, not the physical body part—but the speaker here puns on the dual meaning of the word foot to create a second pun on hand vs. foot.
There may even be a third layer to this pun, since Irish can also have overtones of quick-temperedness and violence; sense B.5 (n.) in the OED entry reads:
colloq. (orig. U.S.). Fieriness of temper; passion, anger, rage. Chiefly with up, esp. in to get one's Irish up.
Considering that the reference here is to a fist blow compared to feet (and thus implicitly to kicks), it may be that Irish here is intended not only to mean ‘paradoxical, illogical’, but also in a second, deeper layer ‘temperamental, violent’.
Best Answer
"How I love the country life" is a pun, because in that context 'country life' has two meanings — life in the country and the margarine brand.
"Those two pears are a pear of green balls" doesn't look like a pun to me, as nothing in there has two meanings.