A guarantee from any limited company will cease to be valid when the firm goes bust because there is no legal body for the customer to pursue their claim against. (You can't take legal action against a firm which doesn't exist. The directors of a failed limited company are not liable for the debts or obligations of that company, so you can't force them to honour the guarantee).
The only exception is when the guarantee is backed by insurance. In which case, the insurers should honour the terms of the guarantee if the firm goes bust.
http://www.bbc.co.uk/northernireland/oyb/goods _services/double_glazing2.shtml
Chris