Wait until Fall, if you want a Windows computer. Seriously. M$ has announced that Windows 7 will ship October 22 this year, and folks, if you must buy a Windows computer this year, you will benefit by waiting on Windows 7 as opposed to buying a Vista-based computer. Note: Vista has improved somewhat, but it’s still bloated, slower than XP or Windows 7 and far, far too resource-intensive, IMO.
So, if you are planning to buy a Windows computer this year, hold onto your computer dollars, if you can, until Fall and buy a Windows 7 based computer. When you do, think carefully about your intended use, though. There will be several consumer-oriented “flavors” of Windows 7 and you’d probably be better-served in the long run to buy the most advanced-featured version that will run well on the hardware you select.
OTOH, if you intend to run another OS on your new computer purchase (or build), Ubuntu 9.04 (in a fresh install only–an upgrade from 8.10 failed horribly for me), PCBSD 7.1 or any number of other OS offerings (I still very much like Puppy Linux for low-spec machines!) would do well on nearly any new computer. In fact, Ubuntu is offering a netbook-tailored version of Ubuntu 9.04 that could make such low-powered machines much more useful than the Windows 7 Starter Edition that M$ is authorizing for installation on netbooks.
Depending on the user’s needs, I could recommend Windows 7, Ubuntu 9.04, PCBSD 7.1 or Puppy Linux 4.2.1. It all depends on what YOU want to do with your computer, what its hardware is, etc.