With young people accounting for more than half of the new HIV infections world-wide, the money will help fund the United Nations' efforts to protect under-25s from the virus in Botswana, Ghana, Uganda and Tanzania.

Infection rates in 15- to 24-year-olds

Botswana: 25% Tanzania and Uganda: 9% Ghana: 2.3%

The UN population fund (UNFPA) will spend the money on youth clinics, Aids education in schools and advertising campaigns directed at young people.

The four nations have been selected because of the willingness shown by their political leaders to tackle Aids.

Safeguard future

Tanzania will receive $16.5m, Uganda $15.3m, Ghana $14m and Botswana $7.9m.

The remaining funds will cover costs for the UNFPA and two non-governmental organisations - the Programme for Appropriate Technology in Health and Pathfinder International.

Bill Gates: History of donations to fight disease

The executive director of the UNFPA, Dr Nafis Sadik, said the grant would put resources where they were most needed.

"The programmes it supports will also serve as models for other hard-hit countries and international aid efforts."

UN statistics show that in some African countries, girls aged between 15 and 19 are up to six times more likely to be HIV-positive than boys their age - an indication that adolescent girls in particular need the information and means to protect themselves against infection, Dr Sadik said.

The UN General Assembly aims to reduce HIV infections by a quarter in the next five years.

Adverse ruling

News of the donation came soon after District Court Judge Thomas Penfield Jackson ruled that Microsoft had broken US anti-trust law by attempting to monopolise the internet browser market.

But correspondents say UN officials do not believe Mr Gates is using them to get favourable publicity - the foundation has a history of contributing to charities fighting Aids and other diseases.

Charitable donations attract generous tax breaks in the United States.