Her firm, Accel Partners, announced a $4.5 million investment in LearnVest, a start-up aimed at women who control household purse strings but are fairly flummoxed by the challenge.

It’s apparently a big market. “Seventy million 18- to 50-year-olds–women–control 80% of household financial decisions, and no one is targeting them,” says Alexa von Tobel, LearnVest’s founder and CEO, whom I caught at her desk at noontime today, bleary-eyed following her own fund-raising feat. (“None of us have slept in 40 hours,” she told me.)

A 26-year-old Florida native and Harvard grad (Class of ’06), von Tobel went to work at Morgan Stanley , in proprietary trading. She soon realized that even as she understood the big numbers there, she (as well as many of her friends and colleagues) were clueless about their own personal finances.

So she quit Wall Street, went to Harvard Business School, and entered a business-plan contest staged by Astia, a Silicon Valley not-for-profit that helps fund women entrepreneurs. She raised $1.1 million and met the right people, including former DailyCandy CEO Catherine Levene and ex-Huffington Post CEO Betsy Morgan, who are now LearnVest advisers. They introduced her to Accel’s Ranzetta.

So what jazzes Ranzetta about this little company? She compares LearnVest, which has attracted 100,000 users since November, to other Accel investments in the consumer space, where the audience initially was small but grew very big. Accel’s wins in this space include Glam Media (targeting women interested in fashion), Etsy (passionate crafters), and Groupon (local deals and coupons). Accel’s best bet has been Facebook. The firm first invested in Facebook in the spring of 2005, when a couple of million college students and twentysomethings were using the site. Facebook now claims more than 400 million active users worldwide.

Now, as LearnVest enters the personal-finance market, some may see an already crowded space–with zillions of personal-finance books and guides and so-called experts, plus sites like Mint.com, which Intuit recently acquired. But Ranzetta and von Tobel see a ripe market in targeting women, in a hip and fun way. LearnVest’s daily emails feature advice such as how to drink wine cheaply without drinking cheap wine, how to save money on your dry-cleaning bill, and how to file your taxes on the day before deadline day.

How to file taxes on April 14…That definitely crosses gender lines.