Hope,
Relax and take a deep breath. I have been dealing with Asset Acceptance for over Three years and you are in a solid position.
First, if the last payment was made any time prior to 4/21/04 then the Statute of Limitation (SOL) is over. Second, they are not licensed to collect in the state of California.
What I did was send a debt validation Letter, several can be found on this website (just click on the link on JCEMT's post)and emphasised that the portion stating that they need to prove the SOL is not expired and that they were licensed to collect in California since the California Secretary of State's web site shows there license as being forfeited. I also sent them a print out of this web page.
They stopped all contact with me.
You want to send this letter right away so that they are prohibited from placing this on your Credit report.
Here is the link for the Secretary of States search page.
kepler.ss.ca.gov/list.html