OK, in order to do this you will need 2 things; the Win2K install CD and the administrator's password. Also, I haven't installed Win2k in a long time and my memory is bad so you may find some differences between what you see and my instructions.
First launch parallels and select the drive image that is giving you problems.
Check the boot order and make sure the OS will be looking for the CD before the hard drive. I don't have a mac in front of me at the moment, but it was under the 'Devices' tab if I recall correctly.
Put the Win2K cd in the drive and start windows booting, it should start booting from the CD and will likely take a while.
Eventually the Windows installation will prompt you to either install, quit or enter the repair console by entering 'R', this is the option you want.
You will then be prompted to select which operating system you want to log in to (there should be only one), then enter the administrator's password. Note that it's not asking you for the adminstrator's name, just the password for the default Adminstrator's account. If you don't have a password, just hit enter.
You should now see a C:\ prompt, type "chkdsk /r" and hit enter. This is the DOS checkdisk and it should look at your drive image as if it were a hard drive (which, for all intents and purposes it is) and repair any inconsistencies. Once it's done it's thing, quit and relaunch Parallels to see if you can log in.
Good luck.
B