Lucky number 7, I guess you could see after the Vista compatibility problems with old hardware and software. Windows 7(formerly known as Blackcomb and Vienna) is the successor of Windows Vista and is at Milestone 1 (M1). Windows 7 Milestone 1 is build off of \\ Windows Vista. Microsoft shipped Windows 7 Milestone 1 to Key Microsoft Partners back in January. The build number of Windows 7 Milestone 1 is very simpler to Windows Vista's. You see here. Vista's RTM build number: 6000.16386.061101-2205 and Windows 7 M1 build number: 6.1.6519.1.071220-1525. After installing Windows 7, Windows 7 M1 installer will automatically create a dual-boot system. So, at start you select Windows 7 or Vista. There are a few changes in Windows 7 Milestone 1. 1, Aero has to be enabled manually. 2, Performance indexer also has to be updated manually. 3, And there are a few incompatibility issues which was attributed to early code. A fully booted Windows 7 M1 system without anything else running will use ~800MB - 950MB RAM. Windows 7 will release at some point between 2010 and 11. First public betas are expected in 2009. There can be any where from 1-5 Milestones, 1-5 betas and 1-3 Release candidates. Milestone 2 (M2) code drop is scheduled for April or May 2008, Milestone 3 (M3) is listed as coming in the third quarter.
Here is a link to illegally download Windows 7 Milestone 1:http://www.live-share.com/files/301743/win7.6519.1.071220-1525.lab_internal.milestone1.iso.rar.html
Here is a video of the successor of Windows 7, Windows 8