My computer is freezing and randomly restarting, what could it be?

I have a HP pavilion a720n desktop. Runs very well for its age and has had no previous issues. Well for months now I have had random freezing that varies. It could happen 2–3 times a day or not happen for a couple days or be gone for a week. I also have random restarts. I will be doing something on my computer such as web browsing or watching a video and all the sudden my computer reboots out of nowhere. It usually only does this a couple times a week. I’ve taken it to several places and they check my hardware, check for viruses, everything was fine last time they checked and it still hasn’t gotten better. I also did a fresh install of XP about 3 months ago and it was fine for maybe three weeks. I am getting very annoyed with this and I cannot figure it out! If anyone can please help me it would be greatly appreciated! Thank-you!
***Also at any given time it says I have 220–440mb available physical memory out of my 1GB. I’m not sure what that means and the number changes a lot can this have anything to do with it? I don’t even have a lot of programs on my computer so why is this number so low?