1) Don't expect to download a tool to fix the problem. The nastiest feature of Antivirus 2009 is that blocks downloads from the major antivirus websites. In particular, Malwarebytes, which is recommended in a number of places to deal with Antivirus 2009, is blocked.
2) Get to a clean system. Just because you can't download the proper tools on a compromised system doesn't mean you can't download them at all. Go to another computer and download the tools you need. Malwarebytes Anti-Malware, mentioned above, can be downloaded here.
3) Send the tools from the clean system to the compromised system. The most obvious way to do this is via a flash drive. However, the version of Antivirus 2009 I dealt with (surprisingly) allowed me to send the mbam-setup.exe program though email.
4) Once the tool (whatever it is) is downloaded, rename it to
Please note that, for step 4 above to work, you might have to restart Windows in safe mode. A description of how to do that can be found here.
Please also note that the above 4 steps (including restarting in safe mode) might not actually work. The version of Antivirus 2009 which got onto my grandmother's computer let me run the antivirus setup program, but blocked the antivirus program itself. My next step after step 4 would have been to create a rescue CD and use that to boot from. However, my brother who also happened to be visiting that weekend had different advice: since my grandmother's computer was brand new, why not reformat the hard drive and just reinstall everything my grandmother wanted? In the end, that's what happened, since I would have been required to go back to my house (across town) to get a rescue CD, while my brother could reformat the hard drive immediately. It's an extreme measure, but I can testify that it certainly worked for my grandmother.
Update: As a potential alternative, I sent a message to Malwarebytes and asked them if they had a link that wouldn't be blocked by Antivirus 2009. They sent me a link, and I added it in this post.