OpacityMonkey
OpacityMonkey is a command-line tool to set the alpha of all pixels in an image to 1.
It uses .NET Framework 2.0, so it should work with Windows 98 and above.
Instructions
In command prompt, type "OpacityMonkey.exe infile.png outfile.png".
Infile and outfile can be named whatever you want, but outfile will be created by OpacityMonkey.
Preferably, use PNG files. The output file is always in PNG format.
Downloads
Drop the EXE file from the zip file wherever you want.
Newer versions will be added to this list if they are made.