Ah, I see now - yes, there is recording of uptime, at least in Windows. Programs like Everest will display it - I'm not sure if there's a way to easily get it without using additional software to what comes with Windows. Presumably the Mac also has such a feature - I'm guessing there would be a Terminal command for it, or there must be utilities that will display it.
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Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)