Could a Microsoft security update make my computer run slowly?

[GypsyCatt]

I have a Compaq laptop running Windows 7 Home Premium and dial-up which, of course, runs slowly, but, about a week ago, my connection started to take almost five minutes just to connect to my Firefox browser. My Yahoo had been running slowly and glitching and now it was even worse.

I tried downloading a new security program, but that didn't help. I didn't add any new programs before this and the only thing I can think of is that there was an automatic Microsoft security update right before this started to happen. Could that be what is making my internet connection start-up run so slowly? If not, what could be doing it?

Thank you.
Let me restate that my computer is not running slowly, it is the internet connection. Also, I have dial-up and so cannot view any videos. This frustrating state of extreme slowness began at a definite time; one day it was slow but not bad, the next it was mired. Yahoo is the worst.