Ok mate well you really haven't got a lot on it then. Windows updates my friend should be set to auto I know they are a pain but everything is targeted these days even Office etc.
Well Windows updates a start but before that do you know if your Avast is up to date and running as you browse?
One or two thins also have you got any other security features on at all.
If you haven't then it might be an idea to download and run Malwarebytes and SuperAntiSpyware both are free and don't take long.
If you haven't had a clear out for a long time try CCleaner it clears the machine of old cookies temp files and all that stuff that will clog up and slow down the works. However if you have sites with passwords then it is an idea to check for cookies for that site and save them. You do that by running Analayse
in CC then go to Options
pick cookies from the list on the left click on the ones you want to keep click on the small arrow in the centre column it will shift to the right and not be deleted. Go back to the brush and Run Cleaner.
If you have never used it this may take some time be patient.
But first try the two security apps just to make sure something hasn't got past Avast - none of the AV's are totally foolproof. Malwarebytes : Free anti-malware download SUPERAntiSpyware | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware!
< free edition
Run these two first and see what comes up. The CCleaner can come next CCleaner - PC Optimization and Cleaning - Free Download
< from piriform site