In NTFS you can manipulate permissions as you want.
- Remove all permissions from that folder.
- Add permissions only to specyfic account.
To do that - go to folder
Security tab and click
Edit. You will figure out next steps.
If other administrator will try to access that folder - he will be notified that he has no permissions, but he will be able to change ownership of that folder.
More correct approach
If you need second administrator to be able to install programs - you don't really need him in "Administrators" group.
Remove second administrator from "Administrators" group and use "Power user" group instead. "Power user" group members can install programs (they have access to Program Files etc), but they cannot control system like Administrator can.
You can create new group and manipulate its permissions in Local Security Policy editor (run -> secpol.msc). There are all existing Windows permissions and you can set them as you want. You can even give "Shutdown" permission for only one user.
Edited. This will not work in Windows 7.
Very secure approach
Use NTFS encryption, as Crippledsmurf suggested in another answer. However - if your disk fails - it will be probably impossible to recover data from it, because there are no data recovery tools (known to me) which can recover encrypted files from damaged disk.
Network server approach
You can move all these folders to another computer where both administrators have no admin rights and create separate accounts for them. Then share these folders as you want.
You may need at least Windows 7 Professional to manipulate permissions like this.