Being forced to sign before paid - help!

What is the name of your state? Florida

I have been doing contract programming work on a internet site that I did not build originally. I am being asked to sign an agreement for work that has already been completed before I receive another check - I have already received two checks and been working without an agreement for three weeks. Part of the agreement states: "Upon final payment, no ownership or other compensation will be due to you from (company name). This is our final agreement and satisfies all other previous agreements. We also hold you harmless with regard to work that you conducted on behalf of (company name)."

I have a previous agreement that states I will receive 2% ownership of this company for my work. I do not care about the ownership. I have two questions: 1. If I sign now, do I have anything to guarantee that I will be paid? 2. Can they sue me for previous errors or defects in the site, even though there is a clause about holding me harmless?

"If I sign now, do I have anything to guarantee that I will be paid?"
*** Based solely on the information in your post, no. But then there are no absolute guarantees of payment anyway.

"Can they sue me for previous errors or defects in the site, even though there is a clause about holding me harmless?"
*** Of course they can sue you. After all, anyone can sue almost anyone else over almost anything. And there is no way that anyone can determine what, if any, liability you may have without reviewing your agreement in full, the work you have performed and any claims that they might make.