Mine did that too. However, you can fix that if you go into your /etc/X11/xorg.conf file and set you resolution in there. I gave mine a few different resolutions so if I'm not on my external monitor, it will use my laptop resolution.
That fixed the tiled issue and also made the lightdm login a lot better looking too. 
Alter this section of that file. Notice the "Modes" section. I added 1920x1200 for my external monitor and 1680x1050 for my laptop. It will try each one until it gets a doable resolution, so when I'm not plugged in, it will jump to 1680x1050:
Code:
# Parallels Screen section
Section "Screen"
Identifier "Parallels Screen"
Device "Parallels Video"
Monitor "Parallels Monitor"
Option "NoMTRR"
SubSection "Display"
Modes "1920x1200" "1680x1050" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Last edited: Jan 26, 2012