Also if I buy a new computer with XP - how long will that last?
Typically MSFT leans on OEMs to stop shipping older OSes a few months after they release a new OS.
When XP shipped I managed to get a few systems from Fujitsu-Siemens which came with both W2k and XP (two sets of OEM CDs) - gave the option of sticking with W2k until the first XP service pack came out, then switching over to XP at no cost.
If I see a similar offer with XP/Vista I'd jump at it.
Many of the machines (about 10000) where I work were still running Windows 2000 till about 9 months ago. Now that we switched to XP, it will be a hard sell for Microsoft to convince us to upgrade yet again. I would image many large companies will not want to spend the money to upgrade to Vista for some time. My company has also hinted about going with open source software for our servers, I wonder when someone in our IT department will bring up open source OS for our desktop and laptops.