It boasts how long it can stay up. Whereas previous versions of Windows were coy about how long they went between boots, XP is positively proud of its stamina.

Go to the Command Prompt in the Accessories menu from the All Programs start button option, and then type 'systeminfo'. The computer will produce a lot of useful info, including the uptime.

If you want to keep these, type 'systeminfo > info.txt'.
This creates a file called info.txt you can look at later with Notepad. (Professional Edition only).