Electricution from a computer is rair but it can happen, I have only had a few small zaps but mostly just from a static discharge.
If you unplug the computer an press the power button a few times most if not all the stored electricity inside the PSU's capacitors will be discharged. Having said that NEVER EVER EVER open the PSU it's self up no matter how long it hasn't had a current flowing through it you could wind up hurt or dead an we wouldnt want that.
But I would do as other have said in this thread, and if you have a older PC to take apart an learn how to put it back together again, get used to installing the OS, Drivers etc an learn how to trouble shoot problems.
Also don't run before you can crawl learn the basics, put them into practice, make sure you know them, then move onto the more advanced topics, then again make sure you know them, an then take if from there.