This might be worth checking out. Don't know if it applies to Win 2000 tho.
Explore/View/Folder Options/File Types, locate 'HTML Document' and click Edit. In the Action list select 'Open' then click Edit.
If the option use DDE is checked and you see a value like "file://%1",,-1,,,,, in the DDE message field, then forget about it.
If you see a value like ,,-1,,,,, in the DDE message field you might try this procedure to see if it fixes your problem:
1. Write down the strings from the DDE fields so you can restore the settings later.
2. Uncheck the DDE box.
3. Click Ok.
Repeat the above steps for the Open in Same Window action, if it is present.
See if this fixed the problem, if not restore the settings you wrote down for both topics.