Freemium is the term that Avast applies to its business model, which offers users a freeware IT security application, but also offers extra features in return for a pay-for software licence.

According to Vince Steckler, the firm's CEO, the freemium approach is already upsetting the traditional anti-virus market.

Instead of paying for advertising or installation on new computers, he says, Avast is growing thanks to word-of-mouth recommendations from the firm's customer base.

This approach, he adds, is the most effective and low-cost form of advertising for the company.

The quiet growth of Avast anti-virus, says the firm, means that the company's products now protects 20% of PCs in the world from malware.

According to chairman Eduard Kucera – who co-founded Avast along with Pavel Baudis in the late 1980s – when the free version of his firm's software was released in the early part of the current decade, they gave themselves room to grow "but never dreamed that the product would gain such global popularity."

As part of the $100 million investment, Scott Collins, Summit Partners' managing director, will get a seat on the board of Avast.