My computer is doing some weird stuff! HELP!

I have a Dell Dimension 2400, it's an older computer, but has worked just fine for me since I got it about 4 years ago. Lately however, some strange things have been happening, like when I save a Microsoft Word document, it saves a backup in the same file. I didn't change any settings, that I know of, it just started doing it. And on my toolbar, I now have the little arrow in the circle to hide the icons down on the right hand side by the time. That was never there before, now it is. But the worst was today, when I started up the computer, everything turned on and booted up normally. I went into Outlook Express to check email, and got that error message box that says "Outlook Express has encountered a problem and has to close." I kept trying to open my email, and wouldn't even get time to select a message or folder before it shut down. About an hour later the same thing happened to Internet explorer, I had to open McAfee from my tool bar and click on a link there to get online. I went to dell tech support via online chat, and the tech had me do a system restore and change the date to a couple days ago, and now everything seems fine. However, I wanted some input on whether or not anyone thinks this may happen again, and why it happened in the first place.

Thanks!!