While it IS easy to *add* a HD to a PC, one of the most difficult jobs is to replace the boot drive, as this involves a re-install of everything. Yes, there are backup programs available, yada yada, but who are we kidding? Home users to NOT back up their entire drives. You're lucky if they backup their most important files (say, like their Quicken/Money databases, address books, Word & Excel docs, etc.) Most users wouldn't know how to back up their whole drives in the correct way so that it could be restored, and far fewer have actually *successfully* performed a full restore.
That isn't to belittle anyone, as this stuff isn't all that easy for a PC tech unless you've got a nice network setup and imaging software (Ghost or Drive Image Pro).
Also, the harddrives at Costco are pretty high capacity and pretty cheap, but they're also SLOW. If you want a high-performance drive, you have to KNOW exactly what you want. That's pretty true with most computer equipment, BTW.
My best advice is to buy a CD-R drive, and BACK UP the data from all of your important applications. Do this at least once a month, on a seperate CD each month. CDs cost less than $1, so you have NO EXCUSE for not doing this.
Even if your drive dies, and you have to get a new one and have your OS reinstalled (not always a bad thing anyway, especially since most PCs come with tons of resource-eating crap installed), then at least you can restore your data and be up and running pretty quickly.
Of course, anything that you feel is critical should also be backed up. If you aren't doing this already, then ask yourself this:
[b]What would I lose if my hard drive died RIGHT THIS MINUTE?[/b] Anything that concerns you needs to be backed up regularly.
As far as advice on what to buy, ask us. We're essentially "disinterested 3rd-parties" who can give you pretty good advice.
For actual hands-on help, find someone privately who will be available to work on your PC long-term. Pay them, and let them know that you will be relying on them if you need help in the future.
Most "free" tech support is worth exactly what you pay for it.
-Troy