This excellent diagram may help...
http://www.diyfaq.org.uk/plumbing/combi.html
The heated water is directed to taps or rads by the Diverter Valve (divertor valve) - Yours may be stuck sending the water in both directions.
A qualified heating engineer will soon have it fixed - ulesss he's from British Gas in which case you will be lured into spending thousands.
Good luck...
Peccavi
June 2009