The uncertainties of middle class man

I need a new computer and it is causing some issues for me. The issue is not over which one I should buy - I am nowhere near that stage yet. The uncertainty is over when I should buy it. My current home system is now getting long in the tooth. True, in its day (four years ago) it was one point something gigahertz of whizzo processing speed and about three quarters of a meg of RAM (after a mid-life upgrade) - so it has lasted well. In fact it works pretty well these days. There are no reliability problems and it runs as sweet as anything. It does everything I ask of it and since Microsoft have finally sorted out all the problems that plagued the early days of XP it can now run really well without crashing. In fact, when I work from home I can have my 20 windows running off the office VPN without any problems at all. So the question remains, why do I want to get an upgrade and why now? Well I blame Microsoft for this. Vista is just around the corner so it I delay the purchase until after the new year I am going to get a new operating system with my new PC whether I want it or not. And I don't want it. Not for a big clock, thank you very much. Why this prejudice? Simple. The old rule says never, never, never become a guinea pig for somebody's new software. So what to do. Buy one now when I really can wait until next year? Answers on a postcard please.