Picked up a virus, but how did I do it?

Guys pc went all crazy and got a virus and wiped out a lot of icons. The virus was wuauclt.exe and PUM Hijack. I am running Avast, and I was just watching youtube and all of a sudden they just popped up. I removed them, but I was just wondering how could I have got them. BTW, avast is updated.
Answer #1
Run a full scan in free malware bytes
http://www.malwarebytes.org/
Answer #2
I’m pretty sure ‘wuauclt.exe’ is not any kind of malware. I noticed it running on my PC a few times. It seems to have something to do with Windows Update.
I was unlucky enough to have gotten a similar virus as you earlier this year. The only difference was I got it from an exe file. However, I was lucky that it got caught by my AV’s heuristics after only deleting approximately 10 icons on my desktop and one .exe file in my Program Files folder. The harm that it caused was easily fixed as I knew what icons/exe file was deleted.
To make sure you have actually gotten rid of the malware that infected your PC, I would run a scan with Malwarebytes as suggested by wazz12.
EDIT: On ‘wuauclt.exe’:
Windows Update AutoUpdate Client. This is a background process which checks with the Microsoft website for updates to the operating system. It shows up on the Task Manager's processes list when it is waiting for a response, such as to confirm permission to download an update.
Source:
http://www.neuber.com/taskmanager/process/wuauclt.exe.html
Answer #3
Thanks fellows.