the name of any window? or a specific one? If you know the name of it, then.. well, you already have it. If you want the "first" window in your hash table, though-- you can't do that.
Your question is quite vague so I'll go over all the ways to get values from hash tables.
Hash tables don't maintain order, they're a data structure similar to your english dictionary. You have a "key" (the english word) and the corresponding "value" (the definition of the word).. you do lookups by the key name, not by the index number. For instance, because you don't (easily) know how many words are between "aardvark" and "arrow", you can't just say "give me the next word", because there is no simple correlation between one key and the next. They're not maintained that way in memory, so its your job to figure out the next key.
If you want to maintain order in a hash table, make the key a number value ranging from 1-n and then have the value contain the rest of the data.
You can also use $hget(htable,N) where N is a value from 1-size. In this case, mIRC will grab the "Nth" key-- but before you go AHA!, know that the order is not guaranteed. It might pull "arrow" at index 1 and "aardvark" at index 2, for instance. Even though you may have entered them in the other way around.
Finally, you can use $hfind to search for a key name if you know part of the name, like "@b*".. again these results are not ordered, but this might be what you're looking for.