'Window server' is not a PC thing - it is used to manage windows on your Mac (the things on screen you interact with, not Microsoft Windows).
You need to be careful with what you assume & deduce by looking at Activity Monitor, avoid killing things that you do not know what they are. Xenophobia is equally bad on a computer as in real life 🙂
The 'PC icon' in your sidebar may have been normal, when a Mac is setup to run 'SMB' (windows file sharing) it can appear as a blue PC in the sidebar. The sidebar shows devices on the same network, these devices 'announce' what services they have active, sometimes they fail to set an appropriate icon and default to generic ones (often the 'PC blue screen' icon). It could also be a PC (or Linux box with windows sharing active) that was on you network too.
Obviously you shouldn't see other devices if you expect no one else to be on the network, in which case reset the wifi password and see if it reappears. It is possible to examine the 'bonjour records' to see what IP & computer is announcing that service, it seems unlikely a 'hacker or intruder' would reveal themselves so easily.
The opinion on Avast around here seems to be that it is a worthless pile of junk. It is often found on download sites that bundle other junk into the installer, it also seems to find many 'false positives'. It is taunting you with suggestions that you have malware installed, but will not say what or where - are they hoping you will upgrade for a fee? Frankly it seems to cause more harm than good. Perhaps I am completely wrong, however you should search here for more opinion on this piece of software.
I wouldn't install Avast on a Mac, the OS already has tools to stop known malware & viruses etc. Excercise some common sense & you can prevent many potential issues –
- Use a non-admin account for day to day usage - be wary of entering your admin password
- don't follow links in email
- avoid 'dubious internet sites' - anywhere that offered paid apps or movies for free etc
- use the App Store for software where possible
- enable Gatekeepers 'app store & signed developers only' option (System Prefs > Security)
- don't download & run apps from just anywhere, especially 'download aggregator' sites (like cnet.com) they can add 'extras' to the installers. The adverts on these sites border on malicious too!
- get apps direct from the developers website, use https where available
- avoid links that direct you to install flash or any other 'plugin'
- be careful with what you connect to the Mac
- be aware the wifi networks can also attack the OS or manipulate your network traffic
- if you are mobile don't connect to any & every access point, a VPN can help make the network a bit safer.
- disable wifi & bluetooth when not in use
If you want an antivirus scanner I'd recommend ClamXAV.
http://www.clamxav.com
It is 'lightweight' - meaning it doesn't attempt to do needless 'cleaning tasks' and it is used by Apple in their email server (in OS X server) for scanning for spam/ malware in email.
I'd also look at adware medic if you want a simple way to look for Genieo other 'adware'
http://adwaremedic.com/
Or read more info on http://www.thesafemac.com
You may find that Avast decides those above pages are a 'threat', this is more of Avast's false matching.
