Forgetful Computer

We need more computer memory for our desktop computer. It is over two years old and has never been upgraded. I believe the only ting we have done to it is add our old hard drive to it and that was no easy chore. There are times when the computer will just freeze up for no reason and it won't even be doing a lot. And it is a pain to get it working again. Most of the time we just leave it on all the time so we don't have to deal with getting it working again. That isn't exactly a cheap solution, but it beats shouting obscenities at the computer to get it working again.