Practically it is impossible to enlarge an image without loosing its quality. In digital world everything boils down to pixels and there is no technology that can enlarge the pixels without loosing fine details. However you will find software that claim to offer loss-less image enlarging capabilities, they do a good job of enlarging the images, but they are not perfect.
Something is better than nothing. Here’s a list of few software that will do the trick of enlarging the images with relatively less quality loss :
1> Vector Magic
If you know any other software do let me know so that I can update the list.