having to reboot before printing

Hi I am new here so I hope you can help.
I am experiencing problems printing over a network, the setup is as follows -
I have a central server running Windows Server 2003, there are about 16 PCs connected to it all running Windows XP Pro, there are 4 rooms with 4 PCs in each room and 1 printer connected via USB to one PC in each room. The printers are of various makes, HP, Epson etc and are all inkjets

When users try to print they sometimes have to log off and back on in order to get the printers to work. This happens more often on some PCs than on others. On several PCs they have to reboot every time they want to print, as you can imagine this very annoying!!!

I have tried installing the printers to a 'local port' on each machine, using the \\PC\printername route, this seems to work for a short time then reverts back to not working. I have also tried installing the drivers onto each individual machine but this has no effect.

Can anyone help as I am pulling my hair out!!