never mind all the guys making fun of u.. tho the situation is really funny but i can somewhat relate to it.
the best option is to -
1. do a multiboot with a blank OS (linux, win98,winxp) such that u dont get the option to boot from or real OS in the boot up sequence. This can be done in may ways.
i) if u have two hdd then it can be changed from bios.
ii) if u have one hdd then use the active partition flag
iii) in case of one hdd Keep a 98/xp bootable disk and when u need to use the system boot using the disk and reset the active partition. in case of multiple hdd change it from the bios
->dont forget to password protect the bios with diff admin and user pass. if ur cousin asks then givethe user pass. he wont be able to change settigs then. if he protests just say u dont knwo what the prob is.
2. Add preventive methods
i) Use some utility to force a shutdown after 20-30 minutes of boot in the blank OS. u can alternatively set it up using even manager and shutdown command.
ii)do not install anything in the blank OS
iii)remove the default OS games too. i would remove notepad and mspaint too
iv) do more stuff like this
3. In ur blank OS use a limited a/c as default and disable the admin a/c. also create a odd name a/c like asppoweruser or winserviceuser etc with a heavy password. now only u know the a/c name and its pass and anyone seeing it wont easily guess that it is a proper a/c
4. In the blank OS(say XP) hide all ur partitions except the blank OS one. basically use software like folderlock or use mycomp> rightclick> manage> diskmanager> right click all partitions one by one and remove the drive letter. Now the normal non-admin user cannot view the partitions not can he change it in the drive manager.
5.The most important part for this elaborate setup to succeed is your acting! you have to behave dumb so that the other guys cant guess what u have done. u have to behave innocent and stupid.
Finish! The -The BlacKCoaT Operative- delivers one again