tiktektak: I'm not sure what exactly you mean by "doing it manually", but the commands you're going to want to use are:
"Levels" (Ctrl+L)
"Hue/Saturation" (Ctrl+U)
"Color Balance" (Ctrl+B)
If you need to separate blacks from dark colours without disrupting the overall look of the picture, "Curves" (Ctrl+M) can also come in handy.
Adjusting the levels of the RGB channel will enable you to improve contrast in an image (also making the picture more saturated in the process), but you should also do it individually for the Red, Green and Blue channels as it's a good way to control the colour tint as you are adjusting the contrast. Certain colour imperfections may not jump out at you, but can become more apparent as you fiddle with the sliders.
Again, with the Hue/Saturation setting, the important part is tweaking the different parts of the colour spectrum individually. Especially with sun faded pictures like this one different colours will be affected in different ways, so you have to fix them individually to get a good result. There's no secret trick to it, really, just go through the colours, one at a time, and adjust the hue, saturation and lightness of each until you're happy. You'll probably going to have to go back and forth between the colours as you go, but it's not difficult to get the hang of.
Colour Balance can be a little bit trickier to use, but it's not always so important. Sometimes it can alleviate issues more easily than hue/saturation adjustments though, so it's definitely something you should play around with. It's not always entirely obvious how you should tweak the picture, so again, just experiment until you start to get decent results.
I usually don't use Curves all that much, it's a little unwieldy, and usually you're not going to benefit from it much. Sometimes it can be useful though, so you should know it's there. The times you are going to need it the most are if you have a really messed up scan where, for example, dark greys are darker than blacks, in which case you can adjust the curves to set things straight. It can be useful in more "normal" situations too, though, as it can give you a bit more control over the contrast than with Levels.
Here's an example of how the above techniques can help you...
The picture could still do with some additional touchups, but my point is that even a scan as poor as that is not beyond salvation.
