of course, you know that the
systeminfo command only works if you have Windows XP Professional, and not in the HOME Edition. The link to which
fastcobra gave you is for a really neat console utility called uptime.exe which has some neat switches to use with it.
if you want a graphical based program to tell you of your uptime, then go and visit this site:
http://rm-f.net/~zdv/code/uptime.html
There, you can download a great little win32 program called UPTIME.EXE and easily see your uptime as well. Either program is good, however, one is a console program while the other is a graphical program.
hope this helps you out.... and enjoy
ps (let us know which one you use... personally, i use both

)