I have to agree with Anubis, a 500mhz computer is great for just using excel and doing basic tasks like homework on. As for Excel running on all versions of windows - Office 2003 requires windows xp so if your going to use that you will have to use Windows Xp.
On a 500mhz system Xp will run fairly well with 256meg of ram, or you could just install windows 98 and everything will be really fast
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A computer like this would be very very cheap and affordable
and thats the main thing