My two cents here is the following. Although use at your own risk.
First thing is to look at Add/Remove programs and see what all is there. Leave drivers and programs you use alone, but some unused programs or programs that require a lot of space you can delete.
Second I would delete Windows Update files if they exist. I have not used 2000 Pro in a while, but it should be in C:\WINNT\Software Distribution\Download. A new computer running XP can accumulate up to 600MB of updates in that folder.
Third, is look at "My Documents" and see what all is yours and not yours.
Fourth, look at all user profiles that are on the disk. Companies often employ multiple users, so there is a chance if networking was not a primary mode (most home systems log on to a profile on the hard disk, but some companies opt for users to connect to a server on Windows that enables them to use any computer in their network) that there could be space used by other accounts. I know account options can be accessed in the Control Panel via "User Accounts." However there is an option to leave the users in and flush out the profile, by right clicking "My Computer" and going over to the Advanced tab. I am not 100% sure how 2000 works, but there should be a User Profiles box and a setting button on it. Clicking it will load a screen with all profiles and their space used.
That is all for now, there are a few more tips but those would be more advanced and time consuming.
Edit: Deleting profiles will break programs that are installed to that profile only.