My current uptime is:
2wks 5days 15hrs 32mins 32secs
My record is:
5wks 5days 9hrs 22mins 40secs
I have a program called Windows Uptime running all the time collecting stats and the average time I reboot my computer is every
two weeks.
I have only once turned off my computer: when I went on holiday for a week. Apart from that it has been running non-stop for 730+ days.
And no, Windows XP isn't any different after 5 weeks of use compared to after it's just been rebooted.
EDIT:
http://www.pcguide.com/care/care/gen/powerOnOff-c.html