Microsoft also dropped me a note about this interesting Windows Vista-related offer, which I assume is US-only:

Microsoft and its partners understand the value of providing small businesses with a variety of affordable solutions that meet their needs.  For small businesses looking to purchase a new PC,  Microsoft is offering an upgrade to Windows Vista Ultimate for $99 with new PC purchase in Best Buy, Circuit City, Staples and Amazons through June.

With the purchase of the Windows Vista Ultimate upgrade offer, customers can also purchase Intuit Quickbooks Pro and/or Windows Live OneCare at a promotional prices during this time.

This holistic offering provides the ultimate small business systems to the small business customers – helping them to get more done faster, work from virtually anywhere, track customers, suppliers, and cash flow, and keep information protected from accidents or theft.

More information: Windows Vista for Small Business