If you wiped the hard drive and completely re-installed Vista and it's still randomly locking up then it sounds more like a hardware thing but just to confirm, you had to re-install all of your programs right? (I want to make sure you went through a full re-install and not just the repair feature of Vista which wouldn't help in this case if the laptop was slow due to software problems).
Assuming you did have to re-install everything then I would first consider replacing the hard drive. Usually with laptops the most common thing to go after general wear & tear (not dropping it...) is the hard drive. The reason I would suspect a hard drive is that it sounds like your drive has "bad sectors" and when Windows try's to write to a bad sector it can cause extreme slowness to a full lockup. Actually happened to me the other month with my Acer laptop. The good news is that you can get a cheap hard drive and they are very easy to install, it's literally a five minute job and hard to screw up.
The process is pretty similar for all laptops and will save you some money instead of paying a repair shop to do it. If you can get a hard drive with a return policy even better, that way you can return it if the problem comes back and it's obviously not the hard drive. Also make sure you get a laptop (2.5") hard drive, as they are smaller than a desktop hard drive. :)