The latest version of Parallels Desktop apparently has the sleazy feature of displaying ads, even though you've paid for the software. Simone Magnelli tweeted a defaults command that you can use to turn them off:
defaults write com.parallels.Parallels\ Desktop ProductPromo.ForcePromoOff -bool YES
Now I don't use Parallels - I use VMware Fusion - and I'm glad I don't. I haven't tested this, so if someone could confirm that it works, and post in the comments, I'd appreciate it.
Mac OS X Hints