In case you need to find how long your computer is up and running there is one useful command which lets you know along with hell lot of other information like system name, Os, Processors detail. However finding from how many hours your computer is turned on is nice just in case somebody is lying to you.
Open up command prompt and type in systeminfo and hit enter. You will see lot of things going in background with a big output including the system up time in it. This works both in Windows XP and Windows Vista.
There are more tips on some features on XP which Rainsazra tells on her post on 20 things you dont know about windows