The method from Tomos is the way to though. You can make new accounts for your friends and then specify per folder or even file which users can access it.
Right-click on a file or folder and select tab security in the window that appears on screen. The content shown in this tab should not require extra explanation. If it does, you are likely going to mess things up.
Still hell bent on using one (non-admin) account for everyone? Then deny access completely for this user. This is dangerous and should not be applied to the main administrator account! Although booting from CD/USB with an O.S. that allows you to read the file-system of your hard disk will fix that quickly, it is a hassle. It also means that you need to access these files with a different account. So, going over left or going over right...the use of multiple Windows accounts will be the way to go.
Expanding on the tricky, but effective method is to create a new Windows user group, called: Restrictive access
Create a new general account that everyone needs to use
Select the file(s), folder(s) or even partition(s) and adjust the settings from tab security to 'Deny Read' and 'Deny Read & execute' for the user group Restricted access.
Then add the general account to user group Restrictive access.
Accessing your file(s)/folder(s)/partition(s) will not be possible. You can be even more bold and use setting 'Deny Full control'. Make a mistake with this and you will be reinstalling Windows, but when you do this right, you will have blocked access in a very absolute manner.
Now I already put several warnings in this post. When applying this Windows does the same, so it cannot be stretched enough, be very sure of what you are doing when using 'Deny'. In a default Windows installation Windows acts (reasonably) friendly, but when applying 'Deny' it will become an enemy before you know it. Choose your fiends wisely, your enemies even more so!