I wasn't able to find an answer to this quickly using Google.  If you have a balloon mortgage and go to refinance, does what you have already paid on the mortgage factor in as far as the 20% down to avoid PMI? Or would you need to come up with 20% of the remaining balance as your down payment?

For example say purchase price was 400k and you've paid down to 300k (25%) does that count, or are you starting over and financing the 300k and need to come up w 60k for your 20%?

Chances will be that when you refi the home it will be appraised to establish the market value. As long as your mortgage amount does not exceed 80% of the value you will avoid pmi.  Or, if your down payment reduces the mortgage to 80% to value than you also can avoid pmi.
Your current adjustable mortgage has nothing to do with your down payment required by the new lender and will be determined by loan amount, credit report, terms, loan to value, etc.

Perfect answer. Thank you sir!

Call a Mortgage company and ask if you can participate in Harp, I already had mine with Wells and they just did it and it was easy.  
If you qualify, you will get good current rates on a fixed term.  They do not reappraise your house with HARP.  

If you do not qualify for harp, a normal refinance will be fine, your house will be appraised and you probably have over 20percent equity, even if you don't getting fixed should be more better.

Call you current mortgage service company and tell them you want a fixed rate, see what they can do for you.