Windows XP

Open Command Prompt and run:
systeminfo

This will return quite a bit of information. Scroll up and find line: System Up Time

Alternately you can run
systeminfo | find “System Up Time”

This will only return the system up time.

To find out the same info for a remote computer on your network run:
systeminfo /s PC-NAME | find “System Up Time”

Windows Vista and Windows 7

Commands above are still valid for Windows 7, except that instead on “System Up Time” you have to use “System Boot Time“. e.g:
systeminfo | find “System Boot Time”

The same information is also displayed in Windows Task Manager, Performance tab, under System > Up Time