"It's estimated that at its peak Conficker infected more than 11 million PCs globally."
-- http://www.sophos.com/en-us/security-news-trends/reports/sec...
Numbers in the range 15-25 million were also tossed around. It's hard to estimate.
Steve Ballmer expected the Windows installed base to cross the 1 billion mark in mid-2008: http://www.zdnet.com/blog/microsoft/microsoft-expect-windows...
Conficker peaked around January 2009 I believe. At 10 million that would be 1% of 1 billion.