desktop everytime, you can't be that short on space that you can't find room for a pc surely.
Laptops are fine if you're going to be using them in more than one location, or as a second pc, but you will get alot more power for your money in a desktop pc.
As for Acer laptops i'd never recommend them, personally if you want a cheap laptop i'd go for a dell, just make sure you do a clean install of xp to get rid of all their added software that they install on their systems.
But have a look around for laptops if that's truly what you want, there's some real bargains around at the minute, Toshiba, Compaq and Sony all make decent laptops.