At five years old, you may want to consider a new computer. I like Dell and they do have some inexpensive but good models. I just installed 4 Dell Vostro 200s at a small shop this past week. They were about $400.- and are Intel Core 2 Duo processors. There should be no problem putting a new HD in your existing computer.
HDs do wear out. They have heads that touch the platters. At some point, something has to give. Like I mentioned, it could also be memory, but that would not cause your HD to fail, but it could cause BSOD and prevent the computer from booting.
My money is on the HD
.
Dave