your plan sounds quite reasonable. Some of the ski areas in the Pyrinees, including Andorra, are not very far from Barcelona and can be perfectly visited in a couple of days. Try to avoid weekends and Semana Santa (Easter) as they get VERY crowded. An alternative to Andorra, a little farther away is Vall D'Aran where is located one of the, suposedly, best ski resorts in Spain: Baqueira Beret. Although this is farther than Andorra, it is quite close to the Toulouse area in France. The scenary and the culture should be more interesting than Andorra (at least to my taste).

As for driving, you have to be careful as snow storms are not uncommon in winter in the mountains. Make sure that, when you rent your car, it has cadenas (I don't know how to say that in English snow chains?). They are mandatory to cross some mountain passes and police (Guardia Civil) DO check if you have them!. Once or twice a year, when there ara major storms, some passes get closed, but usually this does not last more than a few hours.

I'm not a skier myself, so I cannot give you much more detail than that on the ski portion of your trip. However, if you need more information on Barcelona (places to stay, restaurants, things to do, ...) I'll be very glad to help you.

Albert