IIRC if you use a Linux desktop it's really easy... I'm pretty sure that you can just dd the ISO to the USB (you'll need to ensure that the USB is bootable first...). The instructions for PVE (which I have successfully used to create an install USB of PVE; PVE is also based on Debian) should work: http://pve.proxmox.com/wiki/Install_from_USB_Stick
Seeing as it's a old PC though, are you sure that it will support booting from the USB you're using? Even if it technically does support booting from USB I have found some older motherboards can be a bit finiky on whether a USB will work or not... Some seem to only work with 4GB or smaller USBs and even then I have found that some brand USBs will work while others refuse to...
Although having said that, in my experience if you manage to get as far as the bootloader then that probably isn't the problem...
Which OS are you working from?
IIRC if you use a Linux desktop it's really easy... I'm pretty sure that you can just dd the ISO to the USB (you'll need to ensure that the USB is bootable first...). The instructions for PVE (which I have successfully used to create an install USB of PVE; PVE is also based on Debian) should work: http://pve.proxmox.com/wiki/Install_from_USB_Stick
If you are using Windows then another option that should work is: http://www.pendrivelinux.com/yumi-multiboot-usb-creator/
Seeing as it's a old PC though, are you sure that it will support booting from the USB you're using? Even if it technically does support booting from USB I have found some older motherboards can be a bit finiky on whether a USB will work or not... Some seem to only work with 4GB or smaller USBs and even then I have found that some brand USBs will work while others refuse to...
Although having said that, in my experience if you manage to get as far as the bootloader then that probably isn't the problem...