When to buy a new computer?

I am an elderly lady with no more computer knowledge than is necessary to use e-mails and the internet, which is all I need. My desktop computer (Dell) is more than 9 years old, Windows XP, Windows 7. I use Google Chrome. I have a small income, and so do not want to buy a new computer before I have to. But I do not want to risk losing my files or being without a computer even for a short time.

I would like to know: Will my computer gradually die, or will it break down all of a sudden? How bad can I let it get before it is definitely time to replace it?

In the past year or so, it has had the following problems:

After I watch a DVD, the connection to the internet is broken (though it says it is still on), and I must close and restart.
After the computer is on for several hours, ditto.
Everything is slower.
I am getting advertisements, even though I paid for Adblock.
Sometimes, when I turn it on, the screensaver comes up with no icons or Start box, and I must turn it off and restart.
Sometimes it takes a long time to connect to the internet.

All these problems are very minor and do not AT ALL make me want to replace the computer. I just mention them to indicate a deterioration, so you can tell me if this is serious.

Thank you very much, kind people, and apologies for what to you is probably a really simple, boring problem!

MissAdelaide