Realise what? running professional as a dedicated server offering? don't know for sure, but definately not win2k server, unless they did:
1) Have 128 in the machine
2) Install Server
3) Remove 64, leaving only 64.
Have done that, and it works, but win2k server becomes a little slow at responding to net requests. Which is not highly condusive (spelling?) to running a webserver.
Win2k Prof only needs 32Mb, so probably the more likely answer (no company in their right minds would run anything below absolute mins, otherwise their support costs would be higher than the cost of the memory!), and is able to run IIS5 in the same manner as w2k svr but not really it's forte (tuned for someone sitting in front of the box).