On any computer ,set up multiple user accounts. Let one be an administrator account. Password protect it with a strong password.
Let the others be non administrator(or regular) user accounts.These can be password protected also,but just do not let them be admin accounts. Do your internet access with these accounts,never the admin account. Using this method, malware can not be installed without permission. The only way the computer can allow permission is by someone entering the administrator password . Most malware and viruses get installed by the computer user unwittingly via email links or free downloads on the net.Toolbars and freeware often add other software not bargained for.
Also with using this method,you can delete that user account and it shouldn't spread.
And I agree with the above statements, dell and microsoft do not call their customers with a bug report. I hope when you handed over control they didn't peruse any bank account info or passwords on your pc. I would be diligent and every secure account I had somewhere ,go and change your password immediately.
If you left your web browser store passwords for you,you could be at immediate risk.Don't tkae it lightly.
In the future ,I would use a password vault llke Last Pass(www.lastpass.com) to manage passwords. It is a very secure password manager.