Start on the HP. Go into your printer settings (start > Printers and faxes OR Start > Settings > Printers and Faxes) and locate your printer. Right click on the icon and choose properties. Click the sharing tab. Choose Share this Printer. If it is just the 2 computers on the network, choose to let everyone access. Click ok.
Now, if the CD that came with the printer has Vista drivers, put it into the Dell Computer. run the setup program, and follow the prompts to install a network Printer. You should be able to browse the network for your HP computer. If you don't have the CD, or it does not have Vista drivers, Go to Epson's website tp get it.