Replacing a CPU in an HP laptop

The other day my moms laptop gave up some LED codes describing the CPU had failed. I triple checked with HP and they said yes, the CPU needed to be replaced. They gave me a total price of over $400. Ok I thought, ya right!

The pc itself is around 2 years old. (Funny note is my brother bought it for her at Walmart on blackfriday, they sold out of their $300 doorbusters, so the guy at the desk price matched this $900 one.) So its still a very nice laptop, ( I dont have it at my place so I cant give details.) Anyway Id figure the Intel/AMD chip inside could be purchased for around $100 or less today. So I let my mother decide what she wanted to do.

I looked up laptop teardowns for this model and it looked rather tedious, it involved taking a good chunk of it apart. I have taken apart another HP a few years ago, ironically for similar reasons, only to reapply thermal paste due to it getting way too hot. She somehow overheats these even though she keeps them on a desk type stand near the couch. Never on blankets etc. So maybe its an HP thing?

Ok to the point, if I replace the CPU and bolt it back together, will I need to reinstall Vista? Vista is on the pc now. Or can I get by with just loading the current install to get her My Documents off first?