Leaving aside hardware, i have to say that XP is the most stable OS i have ever worked in (although there is tweaking/knowledge involved in getting it there/keeping it healthy.) I didn’t even use a firewall or anything beyond basic antivirus and NEVER had a blue screen in 5+ years of daily use until hardware failure on wireless card.
OS X is nice but i have definitely had problems and some frustrating lack of PC features (like copying the path to a folder from a finder window) – that was disappointing after all the hype. #1 reason i use it is FCP. Also – if you are familiar w/ Adobe shortcuts on PC you will have to start from zero + learn all the Mac ones, which in my experience slows work to a crawl.
Vista – forget about it, total POS. I just don’t understand why anyone would want an OS that does 10% more than XP, sucks up 250% more resources, runs slower and crashes 5x week. I’m sure i could get it into better shape but i only use it on one company-provided laptop and can’t be bothered to put in the effort.