Hi rcdavis welcome to Sevenforums.
Ok, just so I'm clear, you have your printer connected to an XP machine? you want to print from your Windows 7 machine over your network?
No problem. We can do this.
1. Turn on the XP Machine (sounds obvious I know, but so many people forget this)
2. From the XP machine open Command Prompt and type "ipconfig /all" (no quotes) make a note of the IP address. (usually it looks like 192.168.<something>.<something>)
3. From the Windows 7 Machine open "Devices and Printers" and delete all previous failed attempts to install.
4. From the Windows 7 Machine go here:
PIXMA MP780 download product number 146 (assuming your first language is English, if not then pick the one which matches your language, which says "Operating System: Windows Vista 64 bit"
5. Install the drivers you just downloaded onto your Windows 7 machine using this tutorial for maximum compatibility.
http://www.sevenforums.com/tutorials/98073-drivers-install-vista-drivers-windows-7-a.html
6. From the Windows 7 machine check that your PC can see the printer on the XP machine (ie, your network is set up correctly) If you can see the XP machine but not the printer then:
6a. From the XP machine, right click the printer under "Printers and Devices" (or whatever its called on XP) and click properties. Then click Sharing, and "Share this Printer" (or equivalent)
7. From the Windows 7 machine go to "Devices and Printers" and click "Add New Printer"
8. When it asks you what sort of port to use click "Local Port"
9. When it asks you for the port name type (without quotes or <>'s) "\\<Ip address of XP Machine>\<Printer Name on XP Machine (ie what ever Windows see's it as)>
10. Finish the Wizard, making sure to select "Use pre installed drivers" (or something similar, I can't remember the exact wording)
11. Done.