This is charming--and rather clever. It's impossible to tell whether you're making writing mistakes or if it's just the aging house-elf forgetting the Ps and Qs of good grammar. (Sorry, it's the beta in me--always suspicious.
Anyway, I don't mind the rehash of the plot of GoF because your Winky is very appealing and humorous. I'd have even liked more about her interactions with Hermione, as she semed to really resent Hermione's interference back then.
Having her call her masters 'Crouch Senior' and Crouch Junior' is interesting. Sounds like a deliberate attempt to distance herself from her former 'family' or possibly an oficer of the law's habit of describing persons under suspicion by their last name alone--never a first name. It would be too personal.
I chuckled at the colloquilisms that crept into her speech and other funny turns of phrase:
'..we always talk as the third party...' (I guess going to school cured her of that.)
'...my mother punished me mercifully...' (I thought at first that this was a mistake, but remembering things Dobby did to his own head and his fingers by way of self-flagellation, 'mercifully' might possibly be the right term, given that a house-elf left to her own guilty conscience will likely wreak greater havoc on her own body than her mother ever could.
'I am ashamed to say that day I was freed by clothes. ' (Cute)
'...drinking [butterbeer] like Ron Weasley ate his chocolate frogs, which was way too much...' (Hee-hee!)
'...I was hiding behind my eyes, like a little turtle in its shell...' (so endearing)
I like her insights too:
'Mr. Crouch was a hard man who had a hard life...'
'I guess the stress of losing his wife and gaining a son who didn't love him had finally gotten to him. He began to get tired earlier than usual and started misplacing things around the house, then blaming me for stealing them.'
'I remember the day Crouch finally lost it in my opinion...' (LOL)
Thanks for a fun read.