Should your home insurance cover replacing windows if they leaking and beyond repair or do they just cover the consequential damage caused by the leak?
Also if you have been aware of the windows leaking and have put off addressing the damage thereby increasing the cost of the repairs you could be asked to pay a portion towards the repair, in one severe case where subsidence was ignored the company refused to payout on the basis the policy holder did not maintain the property in good repair and should have addressed the problem years back.
Nothing you can do about that, you pay insurance against risk not against faulty workmanship, it is unfortunate that you did not challenge the manufacturers at the time of installation or soon thereafter, it is 4 / 5 years back since they were installed.
Insurance covers damage resulting from one of the insured risk eg storm, flood, escape of water, theft, fire etc. It does not cover maintenance, wear and tear, faulty products or bad workmanship
Check your original receipt/warranty/guarantee some window companies offer 10 years cover but most offer 5 years, you may still be covered. I know you said the company that made them is gone. Many companies sold windows that were in fact made by another so perhaps this avenue may be explored.