if possible, download a 3rd party oc program. and crank the voltage up
oh yeah...here's some more ways not involving opening case
1) like said before, OC the computer till it dies
2) turn the little switch on the psu to 220v...won't turn on anymore =)
3) like said before, flash some random bios onto it
4) If it has video card, flash a supported bios with insane core/mem clocking
5) OC vid card till it dies
Ideas involving opening case
1) take the fan connector out for the cpu fan, do some intense work until cpu fries, plug it back in and tell your boss your computer just shut down randomly.
2) mess with the jumpers on your harddrive
3) steal the cpu =)
some more fun but crazy ideas
1) cut a random wire coming out of your psu
2) More bios messing around with, change bootp priority
3) put some thermal paste on the motherboard