Computer Running Slow - System Crash - Firefox

Lately my computer has been having many issues regarding its performance. It's an old computer, about 4 yrs old. HP Pavillion, Windows XP. It runs at around 100% when I have about 3 or 4 programs open, when I restart I almost always receive a message about hardware monitor finding an error. At one time I was wondering if my mouse or keyboard could be the culprit. Sometimes when I am opening tabs in new windows on firefox, and right click with my mouse.it opens the site in a new window or it shows me the element properties, tries to save the page, bookmark it, etc. I have scanned for viruses and have come up with nothing. I am afraid to turn my computer off because I don't know if it will come back. I've done about three whole system restores on it (all of which have had issues that have resolved if the restore has been started and then let to rest for a few days.)

Another issue last night, was that Firefox would not respond when I entered a website. It would say that it was finding the site, but then it would say "done" very quickly. I put the computer in hibernate (and the part that says "windows is preparing to hibernate" looked drastically different.) This morning I wanted to find out the problem with firefox. i got a message about how firefox had crashed and needed to be restarted. I clicked on "restart firefox" and my computer had a bluescreen error and crashed. This is the message I recieved from windows about that: Blue screen error caused by a device or driver. You received this message because a hardware device, its driver, or related software has caused a blue screen error. This type of error means the computer has shut down abruptly to protect itself from potential data corruption or loss. In this case, we were unable to detect the specific device or driver that caused the problem.