How to Check Last boot up time in Windows and Unix

In most UNIX flavours like AIX the following commands works

Uptime and Who -b
The uptime command prints the current time, the length of time the system has been up, the number of users online, and the load average.

# uptime
10:36am  up 1 day  4:29,  1 user,  load average: 0.02, 0.06, 0.00

# who -b
 system boot  2012-04-03 22:14

In Windows (Windows 7 and Windows 2008)

from the command prompt type systeminfo| find "System Boot Time" 

or just systeminfo without find argument will list detailed verbose information.