Sonds like you might need to reinstall the O/S. If you have a 98se disk that is an easy solution. That will completely reconfigure your system and reinstall any missing files. Takes less than an hour

Is the missing wav file an error message? I would think that you could reinstall just those files if you want to do that

If you have done a clean boot and still can't run defrag, you can try Start, run, type winconfig and click okay. In the dialog box select the startup tab and uncheck any programs like antivirus, quickstarts, etc. You should then be able to run defrag if another program is causing the problem.

Keep in mind that defrag takes a while and at first it may seem to be recycling when it is just doing it's thing.