Venture capitalists invested the most startup funds in the U.S. in six years, despite a slowdown in the nation's economy, according to a new report.

VCs injected $29.4 billion into 3,813 startup companies in 2007, a 10.8 percent percent increase from the previous year, according to the latest MoneyTree Report from PricewaterhouseCoopers and the National Venture Capital Association.

The fastest growing investment sector was for clean-energy companies, which make renewable energy that is safer for the environment. This includes solar, wind and clean coal technologies. The sector raised $2.2 billion, up 47 percent from $1.5 billion in 2006.

Biotechnology and medical device companies raised $9.1 billion, up 20 percent from $7.6 billion a year ago. $5.2 billion of that went to biotechnology firms.

Software companies raised $5.3 billion, to lead all sectors. However in terms of growth it barely grew from $5.1 billion a year ago.

The top five sectors were software, biotechnology, medical devices, industrial and energy business, and telecom firms.