Hi Kiwi Alan,
You're getting a message which says "a valid source for Office 2000 SR1 cannot be found." This suggests that the registry entries relating to Office 2000 are intact, but there is a problem with the executable file and/or an essential DLL file. DLL files are easy to replace, but the executable is another matter.
Have you tried opening Windows Explorer and double-clicking c:\program files\microsoft office\office\excel.exe? If that works (and I'd be surprised if it does) then it means your shortcut is corrupted, and that's very easy to fix. Always eliminate the simplest options first.
If that doesn't work, then Aegis is dead right, and your Office installation is corrupted. And if you don't have the disc, that could be your blooming lot, mate. So then OpenOffice is your best solution.
If you have the time and patience, you could check whether it's just a missing or corrupted DLL file. Download Microsoft Dependency Walker (free download) from
http://www.dependencywalker.com and identify the essential DLL files for Excel.exe. There are only nine or ten. Make a note of any which are missing from your Office folder and/or your Windows System32 folder. Go to a site such as
WWW.DLLDLL.COM and download new ones. Replace the missing one(s), and see if that fixes the problem.
If none were missing, that's where the patience comes in. Try replacing the existing DLL files one by one and see whether that (eventually) fixes the problem.
If it doesn't then you would have to suspect that Excel.exe itself is corrupted. And that's game shot, as they say. Because without the CD there's no way you can replace that legally (unless you buy another copy on eBay).
Aussie Alan