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Maybe you can just move those applications to a separate folder and create an encrypted dmg file using disc utility. That way they won't be able to access or see the folder. When you get your computer back, mount the dmg file and move the applications back to their original locations.

Not sure if it'll work or if it'll screw with the applications but you could try it.
 
It sounds like it's actually your documents/data files used by those apps that you're worried about, rather than the apps themselves. The simplest solution may be to create a second user account and not provide the password for yours.
 
such as iphoto, my mailbox and imovie? by lock i mean set it so that i have to type a password in to open it.

i would like to be able to do this because i have to hand in my computer for a day so that programs can be installed and i have a lot of private stuff on my mac. any suggestions?

An alternate approach is to create an additional account on your computer and set it to administrator privileges. Give the installer that user name and password. They can then use it to install, but will not be able to view or move your data.
 
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