I built a software voice modifier: the "ScarfMod". It's freeware - do with it as you wish.
To download it, just right click on this file and save it to your harddrive. scarfmod.exe.
Here's a test version that might work on Windows 98: scarfmod98.exe.
NOTE: there's currently a bug which causes the program to misinterpret spaces in the filename, so save it somewhere like c:\scarfmod.exe and run it from there.
It requires no setup - just double click to run it. You'll want to plug speakers and a microphone into your computer. The microphone jack is usually pink, the speaker jack usually green.
SCARFMOD has been tried on Windows XP and Windows 2000. It "might" work on Windows 98, but I have no way to find out right now. If it doesn't work on your machine, let me know what you're running and I'll try to figure out how to fix it.
If you're technically minded, there are some ways to tune it. I hope to put a more user friendly way to do this in the next version. You can set the settings by running it from a command window (or a shortcut).
You can drag the saved file into the command window and then add the arguments to the end of the line.
which reduces the modulator frequency to something a little more grating. Increasing the squelch will remove any background noise when you're not speaking.
Here are the usage details if you want to alter the settings:
scarfmod [ -freq:<hz> ] [ -squelch:<vol> ] [ -ears:<vol> ] [ -vol:<vol> ] [ -help ]
-freq modulator frequency. The default is 63Hz
-squelch speech volume threshold at which the modulator cuts in (default 1000)
-ears threshold at which the 'ears' box in the window flashes (default 6000)
-vol volume multiplier in percent, so 100% is the default
-help shows this message
Standard disclaimers apply: This software is provided for entertainment value only - no fitness for any purpose is guaranteed. All responsibility for consequences of failure of the software to perform as claimed are your own.