Hyp's right. There are some optional rules from 3.0 books where movement and tumble rules were different. But from the core books, there is no tumbing while crawiling or tumble checks to stand up witout the attack of opportunity. Even the one above makes it a free action but does not eliminate the attack of opportunity.
For suggestions in this situation, you can pick up your weapon from prone (take the attack of opportunity) and then go on total defensive from prone, or just stand up and take another attack of opportunity. It would depend on how important your dex bonus is, and the guy who knocked you down having combat reflexes and high probability of knocking you down again.
I believe when facing someone with combat reflexes and improved trip, you are pretty much screwed.