Internet (or PC) slows connection speed after Telnet

My friend borrowed my PC and internet connection last week. He enabled and started telnet (both on services.msc and control panel) and started to Telnet. After this I tried downloading MP3 on the internet. But it was so slow! I mean it's really slow that it's not even downloading. I try to download other files. But then it won't download (or is just slow). I use Chrome and tried to use Firefox for download. At first it goes well. But after 2 - 3 downloads, it failed. Also tried in IE but doesn't work. I stopped the Telnet service, disabled it both in services and Control Panel. I also reinstall Chrome. But it doesn't really fix the problem. I am using a download manager (wxDownload Fast) to download files. Yes it download files, but even wxDF had a really hard time downloading a even single 1.6 MB file. I try to ping google and it was 100% completed.

My browsing is okay. I browse Facebook and other sites normally. But whenever I try to play games on Facebook, elements would load but the game would not.
Please help! Thank you very much :)

Add'tl Info:
> We are not using any router. We are connecting directly to the modem.
> Tried to use speedtest. It was normal.

Thanks! It worked! Internet regained it's speed again after scanning laptop for 12 hours. 5 threats found, cleaned and deleted. Thanks!!