Before we begin, I take it we are talking about XP Home or Pro ?
you must have the printer switched of if not the usb cable plugged out. Remove all Epson Software for your printer till no more shows in Add/Remove programmes. Also check their are no Epson icons showing in the Printers folder.
Ok, stick your Epson R300 CD in, Install and then when promted switch on the Epson R300 and carry on to finish of installing.