The Registry key you mentioned exists in XP, but I'm not sure if it existed in 98. I don't have a 98 machine running at the moment, but this tip from mvps.org might help:
Applications can be removed from the Open With dialog box. There is a list of applications stored in a registry key that are removed from the open with list.
Steps to Remove a Program from the Open With... Dialog Box
- Click Start - Run...
- Type regedit into the Run dialog box
- Navigate to the following key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\FileAssociation - In the right hand pane double click the value named KillList
- This will bring up a dialog box with the contents of the KillList. This is a list of applications seperated by semicolans
- Press the End key to place the cursor at the end of the list, type a simi-colon followed by the applications filename and extension.
- This is a sample kill list %1;explorer.exe;dvdplay.exe;mplay32.exe;msohtmed.exe;quikview.exe;rundll32.exe