Have you cleaned virus off machine? (was it a boot sector virus?)
When you say it won't boot - what's the message you get when you try?
Presuming you've got access to another 2k machine (or another machine & 2k install CD), format a floppy and copy ntldr, ntdetect.com - both in root of C: or on CD in \i386 - and boot.ini - also in root of C: to floppy. If no access to 2k machine, create a boot.ini using notepad or other text editor looking like this (assumes 2k on 1st partition of 1st drive):-
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Microsoft Windows 2000 Professional" /fastdetect
Now boot from floppy & see if it will boot your 2k.