I'm being censored. Data going in and out of my computer is being altered. Is there anything I can do to stop it?

You need to be a lot more specific.
Is it a computer you own or one you use for work?
How do you connect to the internet?
What is being censored and how do you know?
Who do you think is censoring things?
There are probably more questions after you answer these so just describe your set-up and what is happening fully.

It's a computer I own. I connect to the internet by cable modem. Orkut captchas, then friend names in twitter, then messages to Mozilla Labs Google Group, then friend names in google plus are being altered. Orkut captchas showed me words that were meaningful to me, that looked like hackers were spying me, friend names in twitter were meaningful too, then I sent messages to Mozilla Labs Google Group and people replied with randomized words I think to show me that they were reading randomized words from me, friend names in google plus were meaningful too. I think the brazilian government is censoring me.

That doesn't sound like censorship, it sounds like something is not working correctly. Censorship would result in missing words and/or pictures and being unable to view or navigate to certain web pages.
As for Orcut capchas having "meaningful words" I don't know what you mean by that but if you mean they were related to things you type about or search for in google it sounds like google "personalisation" to me. You are aware that google monitors your online activity and sometimes uses this to "personalise" search results and adverts?

Way more likely that you have some data issue. Some part of this entire process could be at fault. Testing with uploads and downloads of files and testing against md5 would be a start.

Why would Brazil censor you in that way?