If you want to sabotage your comp so that your dad has no choice (assuming he knows nothing about comps) but to replace:
1. Move/change/delete important files in the windows directory, this includes such things as .dlls, registry files, and key .exe files - make sure you hit your HDD boot sector, so even if he formats, it won\'t boot.
2. If you don\'t care about your hdd (if it\'s small), take it out, scratch it, break a couple pins, use a magnet - whatever, just ruin it, and when your comp won\'t boot, blame it on the whole system.
3. Go and download a "speed dial" that lets you slow down your CPU, then over the next few weeks, have it slowly slow your CPU till you are running at about 10% - your dad will have no choice but to upgrade
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I think 3 is the best idea, cause it involves no permanent damage, so you will have 2 comps after...
1 isn\'t bad, you will probably have to reformat and then reinstall your OS if you really **** up the files.
2.. is.. er.. fun.