PC game software grew 18 percent in 2008, thanks to growing usage of online games that are played primarily on computers and not on consoles.

The PC Gaming Alliance is one of those groups that insist PC gaming is not dying. If you look at U.S. retail game sales, it certainly looks like it is. But much of the growth is in online sales and new kinds of business models that aren’t easy to measure.

In 2009, getting that kind of growth is going to be tough. PC game hardware sales are expected to drop 7 percent, or $1.4 billion, to $18.6 billion from $20 billion in 2008, according to a report yesterday by market researcher Jon Peddie Research. Sales are expected to bounce back to $23 billion in 2010, and the firm expects PC game hardware sales to hit $30 billion by 2013.

According to the PCGA, PC game software revenue was $12.7 billion in 2008, up 18 percent from $10.8 billion in 2007. The group says that in 2008, there were 42 million desktop computers and 31 million notebooks that shipped that, and here’s an important qualification, could be used for PC gaming. By 2013, that desktop figure is expected to grow to 59 million, and the notebook number will grow to 118 million.

Full told, the consumer PC and game hardware market was $68 billion in 2008, and that’s expected to hit $143 billion in 2013.  The installed base of consumer PCs capable of playing games in 2008 was 228 million, and by 2013 that number will hit more than 600 million. The closest rival platforms are the Nintendo DS at 100 million, the Sony PlayStation 2 at 136-million-plus, and the Nintendo Wii at 50 million-plus.

Are consumers actually playing games on those computers? About 60 percent of U.S. consumers over 18 play some kind of games, according to Bob O’Donnell, an analyst at IDC. About 59 percent prefer the PC as their primary game platform, he said.

Software growth was driven by online sales worldwide, particularly in Asia, where PC games dominate in some countries. In China, for instance, game consoles are not legally sanctioned for sale. And in Korea, consoles are virtually nonexistent. David Cole, an analyst at DFC Intelligence, said that China became the No. 1 market in the world for PC games in 2008.

PCGA members include Microsoft, Intel, AMD, Epic, NVIDIA, GameStop and Dell.