Phoebe: I'm... I'm just... I'm the worst person ever. How can I not tell David that I'm seeing Mike

关于window柯林斯词典上有这么一项解释：

If you say that there is a window of opportunity for something, you mean that there is an opportunity to do something but that this opportunity will only last for a short time and so it needs to be taken advantage of quickly (JOURNALISM)

例句：The king said there was now a window of opportunity for peace

也就是说转瞬即逝的机会

语境还不是太清楚，根据给出的内容猜测，Monica那句的意思就是说：其实他那样问了的时候你应该说的。

表述过去的事情，句子却用了过去完成时，这是虚拟语气。就是You should have taken the chance and told him about it

这样的理解下，be one's window不是固定短语，window 是opportunity/chance的意思。

另外

Monica前面就说：也许（应该怪）他没给你机会（告诉他）——注意哦 用了chance这个词

然后听Phoebe说David问过她，只是自己没说，Monica就“Oh，well……”——注意这个有点转折感觉的犹豫

再加上上述词典解释和虚拟语气，得出上述结论。

与之共处

If you work in a company that will finally decide to replace PCs with NCs or NetworkPCs, you have got to live with it

如果你所在的公司最终决定选用nc或netpc，你须与它共处。