I apologize for the confusion, I just took a look at your User Guide for the Deskjet 1280 in section A-1 it lists the compatibility. The Windows 7 OS is not listed, hence the 'not supported' error message you keep getting.
I suggest calling our technical support at 800-474-6836 to see what upgrade options they may have available to you that would be compatible with your Windows 7 OS. If you live outside the US/Canada Region, please click the link below to get the support number for your region. http://www.hp.com/cgi-bin/hpsupport/index.pl
Thanks,