UNIX (and Linux) systems have a command called uptime which tells you when you last rebooted your system. Many people take pride in the fact that they have made their system so stable that they have not had to reboot in x number of days.

Windows doesn't have an uptime command, but it does have two other commands which will tell you your uptime and more. Windows XP Professional users have a command unique to their OS, but even if you don't have XP Pro, there's a command for you.

Windows XP Professional:

XP Pro users have a special command which will output the system's uptime, motherboard chipset, processor information, which hotfixes have been installed, and more. To run it, start up a command window by clicking on Start -> Programs -> Accessories -> Command Prompt. 

In the command prompt window, type in
systeminfo
and press Enter,

The output will rush by, so you can scroll up in the window to view the information, or you can type
systeminfo | more
to have it pause after each screen.

Windows XP / 2000 / 2003:

A user of these operating systems can check their windows uptime, too. Start up a command window by clicking on Start -> Programs -> Accessories -> Command Prompt.

In the command prompt window, type in
net statistics workstation
and press Enter,

The very first line says "Statistics since:"  - this date is when your computer was last booted up. The rest of the information tells you about the network traffic to and from your PC since you booted up Windows.

So that begs the question... what's your Windows uptime? Mine is oh, about 10 hours :) But I've got another PC in the house that's going on 3 weeks... Microsoft patches required the last reboot, and given the frequency of critical Microsoft patches, most PCs shouldn't go for more than a couple of weeks without a reboot.