Hi Ruthless_uk,
I am by no means an expert, but I have built a number of computers for myself. I personally would suggest you do the same. Find a reputable dealer and buy your machine piece by piece. Start with a nice case (ie good cooling) then a damn good PS (850 watts or better). A good MOBO + 6 gig or better of RAM. CPU intel i7 or i9, A decent Video Card (never by the newest - total rip off in terms of cost - but card should have 1 gig of mem on board). Pick your OS (I use Win 7 Home with no problems).Obviously this is all for a PC.
I have never owned a MAC but I fully admit I love the look of that 24" monitor with everything built into it, I mean it is beautiful. What always stops me from buying one (apart from cost) is knowing that I can't upgrade it, whereas a PC is easy to upgrade. Anyway I have 2 computers (well 3 actually) and would welcome a MAC if I could afford it.
As far as viruses go I guess I am lucky - for years and years and years I never used any Virus protection and never was infected - and I would download a lot of stuff from the Web. Truthfully, even if infected its easy enough to reformat and start fresh, which is a good idea anyway - eventually - because I don't know if others are like me but I have a hell of a lot of files that I have left over from who knows what. I have never even defragmented - ever - machine runs fine. I now use a Virus protector and it's biggest gift - so far - is false positives! Ah well.
Laptops are proprietary aswell, your stuck with what you buy - for the most part. But as has already been said, get the biggest screen real estate you can. From my perspective though to get a decent laptop that will run PS well and do some other stuff is gonna cost a fair bit.
Build your own, saves money and it is easy. Check out some forums on build suggestions - particularily for graphics/video usage - there is a look of expertise out there. You should easily be able to build an awesome machine for $1000 - $1400 Cdn (700-800 pds?)