checks the number using a complex set of parameters - the CD does not contain a specific number. I'm surprised you've lost the CD Key - it's printed on a label that's stuck onto the back of the CD jewel case.
Since you're talking about contacting Microsoft for a CD key, your copy would appear to be genuine. Best thing is to contact MS. But if MS says no, there is a workaround. You can create your own bootable backup copy of the CD. During the process, you'll need to copy the contents of the CD to your hard drive first. There's a file in the i386 folder called setupp.ini. Right click the file, select Properties, and untick the Read-Only attribute. Open the file with Notepad. You'll see a line that reads Pid=51873000 or Pid=51873OEM. Change the last 3 digits or characters to 270. Save the changes. This way you will not be prompted to enter the CD key during installation. You can then go on to make your bootable installation backup CD.
This method is used by people who have to install a number of Windows 2000 or reinstall the OS frequently and they don't want the hassle.
There are many sites which give detailed instructions on how to make a bootable Windows 2000 installation CD, usually with SP3 slipstreamed into the CD, so that you don't have to install SP3 separately. For example, click here You need to follow the instructions closely.
You'll need (1) your existing w2k CD; (2) the boot sector for the CD (to make it bootable) which you can download from the link above; (3) the right CD burning software (which allows you to specify certain parameters for the boot sector).