External Apple Monitor causes slow down

I have a MacBook Pro Intel Core 2 Duo 2,53 GHz Cache:6 MB Memory:4 GB Bus Speed:1,07 GHz.
I am using a trial version of Parallels Desktop 5. I have Windows 7 installed on the Virtual Machine. It works fine until I plug in my external Apple Monitor. It connects via 3 plugs-- a charger, a screen plug and a USB connecter. Once I have connected the two together my MacBook slows down to a crawl and all Apps become frustratingly slow. Can anyone help with providing me with the correct settings to use so that this does not happen?
Many Thanks

I have the exact same issue with the full retail version of Parallels 6. Any help would be great. The virtual machine works fine all day at school. However, when I arrive home and plug in my Apple 27 inch cinema display, the entire computer becomes slow and almost unusable - the only solution is to shut down Parallels Desktop which then restores full functionality of the computer.

Attaching a Cinema Display 27" to my MacBook Pro 2009 causes the whole system to freeze. Looks like Parallels 6 cannot cope with the display. Working an a smaller external display works fine.

Funnily, I found a solution that worked for me up to now: I disabled the option of showing the VMs volume on the Mac desktop. I found that running Parallels V6 and TimeMachine starting up caused the issue (a wild guess). Disabling the desktop icon of Parallels V6 stopped TM from trying to backup the virtual machine, I suppose. Now it runs just nicely.