There really is no such thing as an upgrade. It is all the same program, just a price break. So you probably have some printer issues left over from the previous version on the computer you "upgraded" to CS5.1.
Check to see it you have the correct printer listed.
If that looks OK you might reset the preferences and it will clean out any junk that might be a problem. Start PS and IMMEDIATELY hold down the ctrl + alt + shift keys. You should get a reset window. A little tricky, so keep trying if it does not work the first time.