Windows98 doesnt come with "mscoree.dll" so unless you installed and need the Microsoft .NET Framework you can ignore it.
Some diagnostic & antivirus software will throwup this error.
http://support.microsoft.com/?kbid=316091 suggests installing the .net runtimes but since it is unlikely you would be require them and at 24MB it is extreme.
If you cant ignore the error try downloading just the .dll and copy it into your C:\Windows\System32 folder (untested solution) but the .dll itself is tiny.
"mscoree.dll" available here