How to nearly kill a PC

At the weekend I nabbed the memory out of my brother's old PC to put into mine.  Last night I opened up my PC, removed the existing 256mb in the second slot and replaced it with 512mb.  I turned it on and immediately began to smell burning. After turning the machine off quickly, the memory was too hot to touch for a little while - looks like I didn't put the memory in properly.  When I removed it a few of the metal contacts at the bottom were all burnt black and putting the old memory back in the slot didn't work.  The machine seems to not recognise memory in slot three unless there's something in the now burnt slot two so that's useless as well. Now my PC has only one working memory slot instead of three, so instead of upgrading from 768mb to 1.5gb, it's gone down to 512mb.  Fook! Maybe it's time to retire the ageing machine and get a new one...