Ok, this is kind of a long story...it starts with two swedish hitchhiker twins who knocked at my door...not really but it is a long story because I don't know what facts are relevant.
Ok, so I fall off my bike and my 2010 13" Macbook Pro receives blunt trauma. It's in a protective sleeve so I don't think about it. I arrive at school and the computer wakes up from sleep and works, but I can't save anything and its giving me weird hard drive errors. So I restart. Then the computer gets stuck on the Apple start up screen indefinitely. No matter what I do I can't get it to start up.
Everything else works fine. I can run it off of the Mac OSX CD. The guys at the fake Apple store (I live in Israel) can run it off an external hard drive. They tell me it's a hard drive problem and give me a way to high price to fix it.
So I buy my own hard drive. Its a Seagate 750 GB 2.5 inch...blah blah you can see all the details here but I don't think its important.
Luckily I had backed up recently to an external HD. So my sister and I installed the new Hard Drive (easier than I thought!), reinstalled OS X, and then got all my stuff from the back up.
Now everything works well. Or seems to. Until chrome crashes giving me some error about my user profile. Then when I try to open firefox it says it can't be opened because it's already open (which it isn't). Then microsoft word gives me an error that has to do with not enough memory. The I get long stretches of a rotating beach ball while my computer thinks... Then..sometimes...I get a...gasp...Kernel Panic.
I tried running disk warrior. I tried running disk utilities. I tried erasing the hard drive and starting again. I still get crashes. I usually restart before I get to Kernel Panics but I can feel them coming.
I don't see any pattern to when it happens. Sometimes I use my computer for hours straight - streaming music and playing games - with no problem. Sometimes it crashes right away. There are no other problems. I can save files to the HD no problem.
Any ideas? What else can I try?

Applications -> Utilities -Console) and search for disk I/O errors in all the messages. I had those, and it turned out the SATA cable was faulty. I got it replaced and everything is fine now. I got it replaced at an overpriced repair store, but I think I could've easily done it myself. – Bart Arondson Jan 16 at 16:33