So I've figured out the problem with my laptop not starting up is the hard drive (since I can boot from a USB drive).
I've been looking on newegg.com to find a replacement, how do I know it's compatible? I've got an HP Pavilion dv2000. (As a side note, I've had nothing but troubles with this laptop. Sadly, it's not covered by a warranty anymore, but it hasn't worked in more than a year.)
How can I be sure I'm buying the right hard drive? I'm looking for a cheap drive, meaning not too many gb. The computer won't be used to store much, so there's not much use in extra drive space. Also, would I be able to buy a cheap SSD (by cheap, I mean less gb so the overall cost is low, I know they're expensive pieces of hardware). Or does the computer have to be built to use SSD drives?