There are two ways to print from XP Mode on Windows 7:
1. Use the USB attach function to install a local printer directly into XP, or
2. Share the printer from Windows 7.
Option 1 requires you to open Windows XP Mode and click the menu USB > Attach, then install the drivers.
Option 2 is easier, in my opinion, as it auto-shares the printer and drivers, but you have to do a few things first.
1. Under Windows 7, share out the printer. Under Security, make sure "Everyone" has print rights.
2. Open Windows XP Mode. The printer should show up as a networked printer automatically. If it does not, XP Mode may need to be restarted. Go to Start > Run and run the command "shutdown -t 00". Microsoft Virtual PC will automatically restart XP Mode after it shuts down.
3. In Windows XP Mode, right click the printer, go to properties, and try a test page.
At this point, you should be able to simply launch an XP app without going into XP Mode first and the printer will appear.