If that's the case, take a look at "SiteTemplates" folder inside your "TEMPLATE" folder in your 12 hive. You'll see one called BLANKINTERNET. Open it up and click on the "XML" folder and you'll see the onet.xml file, which is the site  definition. If you look at the configuration node with an ID of "0" you'll see that there are several workflow  features that get activated at the Site Collection level. (These are embedded in the "SiteFeatures" node of the XML.) Add those same features  to your own definition. You'll also need to add several feature  id that get activated at the "Web" level as well. (They needed to be added to your "WebFeatures" node in your onet.xml file.) You need to add the features that wll instantiate a Workflow Task list and a Workflow History list.

In general, rather than doing all that work, I would just suggest you use the BLANKINTERNET site definition  as your base rather than AC's, if you're having to add everything back in that he took out. Just my 2 cents.