Found a 'hidden' user account, should i be concerned??

Today i noticed an additional user account whn i went to change my password for my own account (the hidden account not present on the welcome screen where u click your account name). I noticed that the computer was acting more 'sluggish' then usual yesterday despite no virus/adware/attack warnings from the AVG or windows defender or ZoneAlarm installed. The account was named 'AVT.Net.....A' , was set to administrator level and was password protected. Luckily as my account was also to admin level i could simply click 'delete this account' and its (seems to be anyway) gone. I rebooted the computer and did a scan with the security programs but nothings come up. While im thankful nothings happened im concerned as to what exactly did in the first place? None of us are expert computer users and take the normal precautions when downloading stuff and surfing the net and no-one created an additional account

Any insights would be appreciated
thanks:)