It is recommended that GPO PC users have Windows XP installed since Native Instruments (and reputedly Microsoft), no longer support Windows 98/ME. There is a workaround for installing GPO:

WORKAROUND: Find enough free space on one of your hard drives on your computer for about 2 gigabytes of temporary space - you will also need to make sure you have an additional 2 gigabytes somewhere on your system for where you will actually install GPO. Create a temporary folder called GPOdiscs on your hard drive. Copy the contents of the first GPO cd to this folder. Then copy the contents of the second cd to the folder, then copy the third cd to the folder. If you ever get a message saying Windows will need to overwrite an existing file to complete the copy, just say ok and keep going. Copy the GPOLibrary folder and Readme folder from a good installation to your new GPOdiscs folder. If you get any Windows messages saying Windows will need to overwrite a file or folder to complete a copy, press OK every time. Look in your GPOdiscs folder and verify that your GPOLibrary folder is back again now. Finally, in the GPOdiscs folder, double click "GPO Setup.exe". Follow the installation instructions in the installer.. Eventually the installer will ask for you to insert disc 2 or something like that.. just press OK and don't do anything with cds - the installation should continue just fine. Please note that we do not support Win 98/ME and this workround is done at your own risk.