How do I install Windows 2000 to my iMAc?

You're posting in the PowerPC processor-based iMac Forum, so I'm assuming that your iMac doesn't have an Intel processor. If it is a PowerPC model, you can't install Windows 2000, unless you were doing so in conjunction with an old version of VirtualPC. That (now-discontinued) third-party program enabled a Mac's PowerPC processor to emulate the Windows environment. What model iMac do you have and what OS version is it running? Microsoft has marketed Mac-compatible versions of its "Office" software for years, that include Excel. Office 98 Macintosh Edition is the oldest version that would run on a first-generation G3 iMac, and Office X was the first version designed to run on OS X. If you have an Intel processor-based iMac, you should post in the Intel iMac Forum. I would think you'd have problems trying to install Windows 2000 on an Intel iMac, simply because the drivers for more recent hardware (like the Core 2 Duo processor) would not be present in the CAB files.

Most people needing just Excel will not need any version of Windows installed on their Mac. There are several alternatives to Microsoft Office that will work with most Macs. They are all listed on my FAQ*:

http://www.macmaps.com/crossplatform.html

Knowing which Mac operating system you have installed in Apple menu -> About This Mac and which CPU, G3, G4, G5 (all PowerPC, also known as PPC), or Intel Core Solo, Duo, Core2Duo, i3, i5, i7 will tell us which of these alternatives is most likely to work for you.