Laptop suddenly can't handle more than two tasks at a time

Hello, first I'd like to apologize for my broken English, and in case I'm posting this in the wrong section.

Since a week back my laptop's been acting very strange. Visiting one page on its own is slower than usual, but still endurable. However if I have three or more tabs open, I must wait about three minutes for each of them to load properly. Youtube videos above 144p takes at least a minute of buffering, and if I choose to download something, no Internet pages will load until the download's finished. The harder the task, the more time it consumes.

I've done speed tests and they all fall around 1~2 mbit/s. I have sufficient space on my hard drive and 4GB ram. I ran CCleaner twice, as well as Norton AV, AVG & Malwarebytes, but none of these worked. I rebooted my wireless router, tried a clean boot and both did no difference. Task manager doesn't show any change in processes/memory usage from before this problem appeared.

Can somebody give me advice on what to do next?

ShoTTaS said:

With all the troubleshooting you have done, the next thing i am going to do is to check for updates,
if you have updated your windows, you can do a system restore on the day before the problem occurs.

but first, do a backup of your files, just to make sure you don't have any complications after the restore.

Everything is up to date... I'll do a system restore if it comes to that, thank you.

Layback Bear said:

For others to give guidance we/they need your system specs.
Here are two tutorials to help you with that.

By Brink:

System Info - See Your System Specs

By Kari:

Speccy - Publish Snapshot of your System Specs

I filled it out, hope the info is sufficient.

Layback Bear said:

Thank you for filling in your specs.

Now make sure you only have one anti virus install.
Remove all others using their online removal tool.

The programs I use on my system.
Malwarebytes Premium and MSE.

Microsoft Security Essentials - Microsoft Windows

Alright, I now only have MSE installed.