WINE provides an implementation of the Windows API on linux - which is to say that WINE lets you run Windows programmes on linux (with various levels of success). Photoshop CS2 works, Autocad R14 can be made to go, but is iffy round the edges. There are of course alternative applications native to Linux that provide workable compatibility with software designed to run on Windows. High end CAD (let's not forget that an AutoCad license is going to cost you £2000 upwards) is one of the few areas where linux can be found lacking. I've used QCAD at home to work on AutoCad files and it works, but it's not pretty. Perhaps most importantly SketchUp 4 works fine, 5 and 6 are currently dicey (though useable for viewing, editing and saving if you've got nerves of steel) with WINE.
Linux also lacks high-end DTP and Illustration ala InDesign and Illustrator (you'll be well sorted for basic DTP and Illustration) and mid-level video editing ala Premiere/AfterEffects (basic and bleeding-edge stuff won't be a problem) and if you're tied to some proprietary music making software you may also find the switch rather jarring. If you need to do those things you'll be better off with OSX or Windows.
However. It's all rather a moot point as neither Photoshop or AutoCad will run satisfactorily on a 700mhz laptop with 128mb of memory regardless of the operating system you choose. Linux can at the very least provide a satisfactory alternative. There's no gloat, Ubuntu is thoroughly excellent at breathing new life into an aging laptop.
It's not about being anti-MS. We have choices, we pick the ones that best suit our needs. Linux is the most common server OS on the planet - you'll find it and its
UNIX cousins running the Internet, stock exchanges, banking, space exploration and doing mission critical stuff everywhere. It's no big surprise that even a small handful of IT professionals have started to do 'nix.