New to this site and have issues with Windows7 browsers

Hey folks.
Just joined your website. Looks like a nice place. I hope to visit here often. I'm by no means a computer expert. I know the basics and have learned a lot from just asking questions and reading up on certain topics. However, I've got a couple problems that I'm unable to resolve and I'm hoping someone might be able to help.

I've got a desktop computer (Acer) that's about 3 years old. It's equipped with Windows 7. Never had much trouble with it until just recently. I've had the computer reformatted, updated, and it still isn't quite right. When using the Firefox browser, I notice that it sometimes lags when typing an e-mail. It also sometimes lags when I'm scrolling up and down a page. It's not quite as bad since having the computer reformatted but it's still there. I can't understand why it's doing this. Everything is up to date.

Internet Explorer is another browser that I use often and it doesn't lag at all. The only problem I'm finding with this browser is that on occasion it will just disappear while I'm in the middle of doing something. It's almost as if I had exited the browser and was just looking at my startup screen. When I click the mouse button, the webpage I was viewing prior to all of this nonsense will reappear.

I thought by having the computer reformatted that it might get rid of any bugs that might exist and put everything back to normal. However, it doesn't seem like it's done very much. I'm also running Norton and had previously been using Malware bytes. Everything is clean.

I'm just wondering whether any of this sounds normal?. Could it just be a problem with Firefox and IE or does it sound like my computer isn't functioning right?. I've also been suspicious of my internet service provider because I'm supposed to be getting highspeed but it often feels like it's not.

Would appreciate any help.