Isabell: it's a matter of opinion, choosing between those two.  People complain that the Acer build quality isn't that great, but I've heard similar comments about some Dell laptops as well.

On the other hand, Dell is very well known for its after-sales customer support.  I'd say just stick to your budget and do a side-by-side feature comparison between the Acer and Dell laptops in that price range.

As for XP or Vista, you probably won't have much choice there.  Most of the laptops sold today will come with Vista pre-installed.  You could always get a geek friend to install a *cough*legit*cough* copy of XP if you prefer that.

Good luck...