Just as President George W. Bush is launching an ambitious plan to guard against an avian flu pandemic, an administration program to prepare for a potential anthrax attack is running into new and unexpected hurdles. VaxGen Inc., a California biotech firm that last year was awarded an $877.5 million contract to supply a newly invented, and so far unlicensed, anthrax vaccine, acknowledged this week that it won't begin to start deliveries to the federal government until the latter part of next year -- six months later than it originally intended. For months, investigators on both sides of the aisle have expressed concerns that the administration may have invested too big a chunk of the nation's biodefenses in one obscure and relatively untested company. Last year's decision by HHS to award the contract to the little-known VaxGen is being scrutinized by at least two congressional committees. The company's product will have to pass more large-scale tests proving its safety and effectiveness on people before it is fully licensed by the Food and Drug Administration for use on humans, and company officials say they do not expect it to be fully licensed at least until 2007. A New York Times report last December noted that...the company had faced lawsuits filed by investors who claimed VaxGen misinformed them about an AIDS vaccine that the company had heavily promoted but which later failed to work.