Does my laptop have a virus?

[Zoe]

I just got a new laptop and I think I may have accidentally downloaded a virus (I'm not positive how, but I think I downloaded something wrong when I tried to download Mozilla Firefox)
The symptoms include:
-photos don't always load properly on tumblr
-certain pages (tumblr, facebook, twitter) don't load properly at first on Chrome (however, once I refresh the page, the do)
-the same pages very rarely load correctly on Mozilla Firefox
-i tried deleting all cookes from Firefox and reloading these pages but nothing changed
-occasionally, twitter and tumblr won't let me open different pages from the home page
-occasionally when i close my laptop it restarts and then tells me that something unknown caused it to restart

i thought it was maybe a dealcabby virus but there isn't a random.exe file running in my task manager
can anyone help me? my laptop is brand new and i'd like to nip this in the bud if it is a virus. i've run a few different AVG scans but it never comes up with any viruses found. am i just being paranoid?
thanks!
*I've used AVG for years and it's never been a problem for me, I don't think it's the virus now, and Internet explorer is the only browser not having any issues