I suddenly started lagging in TF2, whereas I've never lagged before. I then logged onto LoL, played a couple games, and now I'm getting spikes in LoL aswell.

I've used the free version of AVG to search for viruses, cleaned up in my files, used CCcleaner, defragged my comp. Dunno what to do.

If you can help me solve my issue, I will reward 200 fg between the ones who helped me.

Try going to a speedtest website to see what speeds you are getting, and post them here please.
I recommend speedtest.net - list the ping, download, and upload please.

Did that already. 20mb download, 5 mb upload.

Did it give you a ping result by chance?
Can you try running a trace route? This basically sends packets to a website and shows you every stop that it makes to get to the website. It will help see if the problem is with your ISP.
To do this launch your command prompt (start - run - type cmd/press enter)
In the black box type in the following exactly as I have it written: tracert google.com

This will take a minute or two to complete. Once it's finished, could you take a screenshot of the results for me? This will at least tell us if the problem is locally (computer, router, ect) or a problem with the ISP.

You should try WinMTR and ping LoL/Tf2 servers. Other than that, I think many people have issues with online nowadays, I sure do and so does my friends.

Try to use Hijackthis to see what's running in the backround.
Download -
http://www.filehippo.com/download_hijackthis

You will need to do a scan with a logfile witch you put here
http://www.hijackthis.de/

Then it will show you what programs running in the backround and you might something nasty there.

May I ask what sort of router you are using, and how old it is? Plus post up your ping as Creations said.

My ping is perfectly fine. The lag's gone now aswell, without really having done anything.

I noticed you're Danish; I know TDC have had some issues lately, so this might as well could be it. My friends back in Denmark also ended up getting intermittent heavy lag spikes yesterday when we were playing and the day before yesterday. They live near Copenhagen.

TDC used to be responsible for most internet related cables in Denmark and has later sold parts (cable wise) of Denmark to smaller companies. Your best bet would be to call them - or the company that is responsible for your connection (ISP) - and tell them to monitor your connection for a week's time, to see if this issue continues. This should free by the way.

If they find the issue is from their end, they might switch you to another port or even send a technician out. They did that for me, after lightning had struck our house's backyard, which resulted in random disconnections. Anyway, just a heads up!

fist you need to unistall LoL, then fix the problem, then get a better game