What about disabling CPU cooling until it would run slow for throttling or would totally freeze ? SpeedFan is not opensource but maybe you'd be able to find a tool like that, having support for different mainboard chip and knowing how to turn fan into software-controlled mode and stop it ?
Also there are overclocking tools like SetFSB and AMD OverDrive and MSI AfterBurner and ...
Just push your CPU or RAM over the limits so it would actually generate random errors.
Another thing would be some denial-of-service attacks onto some Windows resource exhaustion. Just think of some invisible process, running threads with "critical" priority on every CPU core in infinite loops, making other processes draaaaag. Combine it with fork-bomb concept, so they would not be easily killed from Task Manager. Also if each of them would be open and lock a hundred od random files it would be a bonus. I remember Windows 2000 crashed just because a program runned amok kept opening files until some critical Windows server could not open the file it needed
I also wonder if good old AT Keyboard chip is still emulated in its full capabilities on modern PCs, including programmatically pressing Reset. Google about i8042 and ports 0x60 and 0x64. Then you would have to run some drivers allowing for applications to hardware access, like the one installed by SpeedFan or PowerStrip. Hmmm.... what if those chip would allow you to re-program DMA Controller to do a random wiping of RAM content ? Sooner or later you would by chance destroy critical OS data. OTOH before you do it, you may damage some cache buffers so maybe OS would damage HDD content
Now... if you foe is a dummy, you can try yet another trick: make window having no desktop/taskbar. It is nothing for a slightly experienced user, but might confuse dummies.
- you make copies of EXPLORER.EXE into your custom name looking not-suspicious but not resembling any usual tern like "explorer" "shell" etc.
- you make windows run that new copy instead of explorer.exe - that can be done by SHELL= parameter in WIN.INI or in registry. Google for custom shells.
- after reboot you delete old EXPLORER.EXE (all the occurences) or replace them with CMD.EXE or some screensaver. SFC (windows file protection) would try to revert that change, you would have to tame it one way or another. Usually it can be done by changing also EXPLORER.EXE in WFP cache
- by windows scheduler you just kill that process of renamed explorer and keep killing it if it would automatically restart itself. User would not be able to restart it manually as the EXPLORER.EXE file was substituted and perhaps he would not know the real name.
However for a more or less experienced person running CMD or task manager is enough to launch any program he would need.