C840 no boot

I recently found out (from a techshop) that my laptop, C840, has a bad motherboard. That's what the shop's guys told me anyways. It would sometimes load up the brand new HDD i put in it (the operating system and everything)... other times it would say "no bootable devices" They told me a new MB would cost me 300+ dollars... *rolls eyes* so i ran home, hopped on ebay, found a "guarenteed to work" motherboard and bottom plastic (the laptop w/o ne of the hardware), swapped in everything into the new laptop, and now when i power it up, the green lights turn on for about 3 seconds then shut off. The fans or hard drive does NOT turn on/spin, and all i hear is a small amount of noise coming from the speakers when i hit the power button, much like a little "pop"

I've tried flopping the memory sticks around, tried only using 1 of them in each slot... i've ran out of ideas! and i feel horrible because i was buying this motherboard to fix my uncle's computer. Im worse off now from what i was before.

I was going through some threads and found my previous issue (the "no bootable devices" issue) and people said it was a failing HDD. I find that somewhat hard to believe but i have heard of faulty HDD's that come in the mail new. It's a Toshiba MK6021GAS (60 gig) btw. I've never been a fan of Toshiba HDDs but they had the best price at the time. Maybe that's where all these problems originated. idk. all i know is im getting a huge headache.